The metagame can change without any external influence. By this I mean, there are clear, persistent, and strong trends in the metagame that happen organically without any tier drops, new move tutor moves, or anything of that sort. One example is Latias, which was A+ rank before UUPL really started to get going. It became S rank when people stopped using "inferior" (they aren't bad, just not optimal) sets in Defog and started to use Z Latias sets. Another example is Conkeldurr at the same time, when people didn't think it was even B+ rank worthy, until the power of Flame Orb sets discovered.
When Zygarde was at C-, it was seeing next to no usage in major games and was basically abandoned because nobody could use its best set, Choice Band, effectively. However, this was a mistake on the playerbase's part because Zygarde, as I will argue later, had at least C+ potential, just nobody knew about it yet. This potential comes from abusing it on Spikes offense, which was not previously utilized. So already, when I ranked it before at C-, it was underrated. Just nobody knew about it, like nobody knew about Flame Orb Conkeldurr or Z Venomoth in previous metagames.
And then I noticed a few posts in this thread saying Zygarde was getting better, which made sense because it had a favorable matchup against a majoity of the drops. Additionally, I lost a few games on the ladder to it when using Volt-Turn and noticed how strong it was when paired with Spikes against my stall team, even losing a game.
So at this point, I understood Zygarde was previously underrated when at C- and even just became better, relative to its previous "true" ranking of C+ or B-, because of the drops.
Finally, I noticed someone post a nomination from C- all the way to A- for Zygarde, which is obviously huge, but it wasn't as if their reasoning was wrong. I decided to compromise and make it B+, after noticing the other nominations were B-.
Then I posted the update and upset many of you. Sorry.
Reading many of your posts made me doubt myself in the ranking, so I decided to make my own Zygarde team (I did not even use Zygarde prior to the last update) with Spikes.
You can search my replays under "Zygarde for B+" I saved all but a handful, after playing 63 games total because somebody complained to me that my previous experience was too small to be statistically significant (by the way, we aren't running statistical tests here so...). Here are a few of my results, representative of Zygarde's abilities.
After playing these games, I am more convinced that Zygarde should remain B+.
But I did read all your posts and play 60+ games because I feel very much like this is your community project so you deserve a say. Let me tell you why your arguments don't convince me:
5 ranks is too much of a rise?
It's big, but I explained before it was underrated. There is no point in doing small drops for the sake of small drops when the evidence suggests a different ranking.
Zygarde is too weak defensively?
You should think of Zygarde like Weavile - incredibly hard to switch into offensively, easy to use STAB, and a great speed tier. Weavile is also weak defensively. But it still succeeds overall; in any case you guys misrepresent Zygarde's defenses as it can switch into Rotom Wash sometimes and Nihilego and resisted moves from Nidoking and Terrakion and others. It's bad, but not Sharpedo.
Zygarde is too weak offensively?
You guys are probably both underestimating how useful Spikes support is and the power of Choice Band in general, for example it gets an easy 2HKO on max HP Mega Latias with SR. SR means nearly all offensive threats cannot switch in. Spikes too means a good portion is just OHKOd.