I disagree with you for two reasons:I know there's been a lot of debate about whether we're getting all the old legendaries back, but the more I look at things and think about it, I'm honestly inclined to believe that this time we aren't going to get every single legendary usable in SV.
I know the Shaymin isn't in the scrubbed Dex entries list and is known to eventually become usable, but back with the Crown Tundra all of the old legendaries were in the scrubbed dex entry list and the fact that all the old legendaries were returning, chief among them the major box legendaries of the Mewtwo archetype, was very heavily advertised in official trailers throughout the pre-release period.
This time most legendaries aren't in the scrubbed dex entry list, but more importantly, there has been no advertisement officially of old legendaries coming back thus far. What has been advertised, however, is the fact that Indigo Disk will have every single starter Pokemon (or as they like to officially call them, "first partner Pokemon"). The datamined list of scrubbed Dex entries of returning Pokemon has indicated that each and every single starter Pokemon from the previous generations, from Bulbasaur, Charmander, and Squirtle, to Grookey, Scorbunny, and Sobble, is returning in the Indigo Disk, and one of the recent trailers has really highlighted the fact that all the starters are back.
Which is to say, I think the fact that we have every starter in Indigo Disk is what will be SV DLC's selling point, and what they want to highlight this time around as opposed to SwSh DLC bringing back all the legendaries.
In that sense SwSh and SV go for opposite directions, and together complement each other as to which special groups of Pokemon they have in their Part 2 DLCs.
SwSh had all the legendaries, but not all the starters (only three old-gen starter trios made it)
SV will have all the starters, but not all the legendaries (some are HOME transferrable and may be obtainable somehow, and some may be added, like Shaymin, but ultimately it will still only amount to some but not all).
This is what I think is going to be the situation.
1. GS Cup. GS Cup has been a staple of the VGC circuit since 2016 and has been in Gens 4, 6, 7, & 8. The fact that the only in person World Championship for Gen 8 was GS Cup speaks volumes as to how much TPCI values the format. I highly doubt TPCI would allow Game Freak to buck that trend unless they absolutely needed to.
2. Marketing. Dynamax Adventures was revealed in a trailer that was entirely dedicated to The Crown Tundra. This was done to ensure the most amount of hype for it. It is entirely likely that Game Freak is waiting until after The Teal Mask releases to advertise the legendaries returning in The Indigo Disk. Right now they don't want to overshadow The Teal Mask with a huge announcement such as all returning legendaries. Especially if they potentially play a plot significant reason.