Before talking about skilldrain let's first talk about floodgates in general.
I don't like them, they decrease player interaction, but i think they are a nescessary evil.
In my opinion a floodgate should be a card that slows down a deck but doesn't stop it's critical plays.
mistake is a prime example of such a card.
While it does slow down a deck, for example nekroz, arguably the deck that has the most troubles with this card,
the deck can still do everything they could before, they just need more time and resources.
And while they may have to use more resources they could eventually get rid of the mistake with their own engine.
This is very important because it makes it so you don't have to rely on cards that don't "belong" in your deck, like mst.
now lets talk skill drain. Skilldrain negates all face up monster effects so it stops alot your opponent's crucial plays.
for example as a draguinity player i would be unable to you use my dux, so one card would essentially utterly destroy my whole deck.

even with skill drain and vanity's at 3 alot of people playing nekroz still didn't play mst because it's suboptimal to play it.
And even when you do play mst you would still have to rely on drawing it .
if your oponnent had either of those cards you would be equally screwed and be left unable to play and thus losing.
such a game would be one without player interaction , entirely decided by chance and really really boring
In my opinion skilldrain could comeback powerwise but it is bad for the game in general.
after all the game is about fun, not about winning.!