Firstly, thank you for creating this thread, Kevin. I remember initially being of the mindset that I did not want Latias in the tier because 1) I didn't think Latias was good enough to be brought back, and 2) I thought it encouraged the use of Dug even more since there was an additional need for steel types to answer to specs draco meteor, and I also felt that CM Lati-Dug teams would be very threatening.
I was wrong on the first count for two reasons - 1) Something not being good enough to be brought back shouldn't be a valid basis for tiering, and 2) Latias is an excellent mon that clearly was good enough to be back. On the second count, when speaking with you in PMs, you told me something very important that realized to me that Dug was the issue, not Latias. You said that the very same teams that would be lose to Dug-Latias would be the same that would be weak to Dug-Cress, and in the back of my mind, I always wanted my teams to be good against Dug-Cress combo. This made me realize that it was Dug that helped something like Cress become better, or basically give something like Cress the ability to sweep a team entirely.
I will address my issues regarding Dugtrio with the following points:
1) Constriction in team building
I can't vouch for all the other SPL DPP Players, but I personally feel like I have gone out of my way to make sure that I am good against Dug-CM Clefable teams (or other kinds of support) because of how good they've gotten all of a sudden lately. There are two ways I can go about this:
a) Not run pokemon that can get trapped by Dug as an answer to a threat, and
b) If I do run pokemon that get trapped by Dug, make sure that I'll pair myself with enough answers to punish the Dugtrio user
a) The first approach isn't really a healthy approach, because it holds me back in a lot of ways. For example, let's say my answer to a Zapdos is something like Tyranitar. In order to avoid not getting trapped by Dug and while simultaneously being able to answer something like Zap, I could run something such as spdef Rotom, Latias, or Swampert. Of course none of these are guaranteed answers, but they're simply an example of pokemon that are capable of beating Zapdos while not being weak to Dug. The issue here is that if my team badly needs a weatherbreaker, for something like rain, then my need for TTar is so huge that by running like Spdef Rotom / Latias / Swampert, I am making myself weaker to a rain-oriented team. Another example of this would be something like SD Scizor. I love using Lefties Heatran with either Taunt or Roar as an answer to SD Sciz. The tier has multiple options to wall SD Sciz that aren't trappable by Dug - like Bold Rotom / Impish Fire Fang Glisc / Heat Wave Zap. So if I run lefties Heatran as an answer to SD Sciz, I could be screwed if it gets trapped by Dug. An example of this would be KG's team vs Shawyu, and luckily there was a Fire Fang Gliscor in the back as a back-up. This is basically the constriction in teambuilding that I am facing, where I don't want my defensive answers to be trappable by Dug. Osgoode ran Curse Gastro for the purpose of being answer to CM Fable + Dugtrio simultaneously. So my current mentality when building is that my defensive answer should be able to answer to threat X + not being weak to Dug.
b) I don't think Dug-oriented teams are invincible, they will have some flaws or the other. If you overload your team with multiple answers to abuse a Dugtrio, I think it will be harder for such Dug-oriented teams to keep up with such threats. There are two issues with this approach for me:
i) By overpreparing your team to potentially abuse Dugtrio, you could be creating a defensive hole in your team with respect to other common threats in the metagame, like Starmie.
Realistically speaking, some of the best Starmie answers are actually grounded pokemon like Passho TTar / Spdef Jirachi, and now thankfully we have Latias at least. But the point being that while you may be good against Dug-oriented teams, you can become weak to generic teams. So this becomes a situation where you have to pick and choose what playstyle to be weak to.
ii) You kind of have to pray that you have the right offensive mons to abuse Dugtrio
A list of pokemon that can abuse Dugtrio - Gliscor / Gengar / Rotom / Dragonite / Breloom / Latias / Gyarados / Bronzong. I don't know if a Dugtrio-oriented team has been built to handle all of such threats, but I am perfectly sure that these Dug-oriented teams are built to handle some of these threats; there's just an uncertainty regarding which threats are not covered. Even then, not all of these pokemon are 100% going to shift momentum against the Dugtrio user. Something like Gliscor needs SD to be threatening vs CM Clefable, Rotom requires Scarf to punish more defensive teams that have Clefable. Dragonite is one of the scariest answers in that list. Breloom to me is the second closest on that list. I feel like Spdef Jirachi can handle Lati + variants of Gengar, although it still has to be wary of the trick from Lati or WoW / DB from Gengar. CB Gyarados can be contained by Milotic, while DD Gyara can be handled by CM Fable. Bronzong is going to have a hard time breaking through Skarmory or Bold Rotom, and vs something like Hippo, it will need explosion. I have given myself the benefit of the doubt here where I've listed the scenarios as ideally as possible it would be for my argument, but the point I want to get across is that such Dug-oriented teams do try to make sure that they are answerable to such deadly threats, so it isn't like just having these offensive threats on your teams makes you solid against Dug-oriented teams.
2) Risk vs Reward
You're right when you say that Dugtrio is a high reward pokemon, but I want to elaborate more on the risks that I see with using Dug. To me, the risks associated with Dugtrio trapping are:
i) its ability to come in to trap a threat;
ii) its ability to trap said threat (whether that threat is carrying scarf or shuca)
iii) the ability to be punished for being choice-locked -> pursuit / AA / EQ
i) U-Turn into Dugtrio has become a very prominent strategy for Dugtrio to come in and safely trap a threat. Off the top of my head, I can think of KG's SD Scizor U-Turning into a Heatran, and it being trapped by Dug; then Bab's Jirachi U-turning into a Heatran and trapping it with Dug. Dugtrio has decent enough bulk to switch in on an Iron Head from Jirachi and take it out then too. Dugtrio can also come in on a Fire Blast from Tyranitar (an example of this would be if a Tyranitar faces a Skarmory), but risky nonetheless. Out of those all, I'd say U-Turn gives Dugtrio the safest pathway to be on field.
ii) There is a risk that something like non-lefties Heatran could be Scarf, but it is possible to perform a calc or two that would clear up your doubt as to whether that Heatran is scarf. There is also this same risk with trapping something like Shuca TTar or Shuca Jirachi (if revealed to not be scarf). I do also think that if your objective is take out TTar or Jirachi, then being able to incur has much damage as possible on those mons is beneficial to you because the lack of lefties + rocks damage means that they won't be able to switch in constantly to handle whatever threat you have remaining.
iii) I addressed the weaknesses associated with EQ-locked in the previous paragraph, but I do agree that there is a much higher risk when Dug is locked into Pursuit / AA because it also allows other powerhouses like SD Lucario to possibly clean through.
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I may have missed a point or two that I wanted to get across, but I think this is good enough to start the discussion.