How can you even type that there's not enough data when Twin has been a major archetype in Modern from the format's inception all the way until it's banning?
Being a consistent and successful archetype in a format is not justification for a banning in an "eternal" format, unless you're advocating the implementation of adjudicated rotation via the banlist. In order to justify a banning, you need to demonstrate that the archetype overperforms (that it consistently converts above its representation rate into Days 2/Top 8s) or that it is overrepresented (that it takes up too high of a percentage of the metagame). The data did not support such a claim as of the time that Splinter Twin was banned.
This is basically what you say when the evidence is against you and you want to get off your unsupportable old theory without specifically admitting you were wrong. "Twin isn't a good ban unless you also unban Visions" is the same actual claim as "Twin is a good ban as long as you also unban Visions," really.
Except they weren't done together. Twin was banned as a singular action; Vision happened later. Perhaps they were/are both part of a larger strategic initiative, but we can't know if that's true. Each move on the ban list has to be judged on its own merits, and should be judged based on the data and information available at the time. Metagames are unstable systems with a large amount of factors influencing their direction which make them very difficult to predict. I believe you need large amounts of data compelling you to make a ban, and that data did not exist at the time that Splinter Twin was unbanned. Otherwise, you're taking a gamble that your predictive model of the metagame is correct, as opposed to making a move based on data.
Now, I also believe (and this is just part of my philosophy on how you should manage a ban list) that the banlist should be as small as possible, so any move which results in a net decrease in cards on the banlist is likely to be a good one (this is really just a heuristic for finding which cards are the most dangerous). If banning Splinter Twin was part of an initiative to pull more cards off of the banlist, then I would support that move on that basis. Ancestral Vision would be one of the cards that I would imagine was being suppressed by Twin, and there are more still on there that could theoretically come off now (one of Ponder or Preordain, for example, could be tested). Sword of the Meek is an orthogonal win condition for a control deck, and isn't really part of this discussion, so the jury's still out here.
I think what frustrates me most is that because of the way that this all went down, we have no idea what the effect the Twin ban would have had on its own. It was banned at the start of Eldrazi Winter and Sword of the Meek/Vision came off when Eye was banned. We never got a clean look at the specific impact of the Twin ban; it was just the start of a tidal wave of changes that cleaned out the metagame. This goes back to the point I've made before that you shouldn't update the banlist at the same time that you release new cards, but it's clear they aren't going to change that.
All that to say this: I don't believe the banlist update with OGW was correct. I believe the banlist update with SOI rectified a lot of those mistakes. I think we ended up with a better Modern by the end of all of it, but I don't believe they handled the situation correctly. Especially we had just gotten over the "whose favorite deck in Modern is getting banned this time" meme.
