Mechanically, it seems like IGAU and I2 are very similar. That doesn't seem to be a problem to me, and it seems like people are fine with that. But I notice a pretty serious issue here. If this has been brought up before, or if it seems redundant, or if it doesn't belong here, just delete me forever while I beg for your sweet forgiveness.
If you look at other fighting games, a legacy character (like Ryu in SF, for example) is generally designed to be playable to anyone who has played him before, regardless of which game they played him in. But look at the differences in those games. Each iteration of SF is massively different. If you saw Daigo play Ryu in SF4, you're still interested to see what he might do with Ryu in SF5 because the game is totally different (in this case worse, but still different).
Personally, I greatly prefer I2 to any version of SF, and I think the decision to leave the game mechanics largely the same as IGAU is a good one. But it's a unique problem, because now you have a number of characters (Adam/Aquaman being the biggest offenders) that are very similar to their IGAU versions, and a game that is largely the same as the first one as well. This might not be a problem if these characters sucked. But they don't suck, they're the absolute top of the tier list.
So who is to blame? You can't blame the players, because they are just picking characters that can win. You can't blame the audience for the generally muted reaction (or outright boos) when another pro picks Aquaman (or whoever), because they're understandably bored of these characters after years of playing/watching them. So who do you blame? I guess ultimately it's NRS' "fault", but it's not that hard to fix, and the game outside of these few characters is (in my opinion) pretty well balanced.
So I guess the solution is sort of obvious..
TLDR: Essentially when we see a Top 8 full of Aquaman/Adam/Superman in 2017, it's too similar to the game people watched for most of the duration of IGAU. The game is largely the same (which is fine), and the (legacy) characters are basically the same (which might be fine if the game itself was different). Same game, same characters = boring. But it's an easy problem to fix, because this doesn't apply to that many characters. Unfortunately, the characters it does apply to are the strongest in the game..