You cut twice in Seniors. I'm sorry, but that does not make your argument viable. Any semi-good player can do well in Seniors, deck-independent.
Yes, pretty much every good deck can kill a Mew T2. ZPST, Thunderus in Eel decks, Cobalion/Terraki in CaKE, and yeah that's about all the good stuff now. Except Chandelure, but your deck just fails to Chandelure even though they can't kill Mews early. Having that auto-loss is another thing that makes the deck bad.
Kyurem and Yanmega are hugely popular (Yanmega to a lesser extent, but Kyurem to a great extent.) And FSL is great and all, but you're left vulnerable for a turn if you have to use that as your supporter and you don't have Mews out. That's another prize for me.
It's very true the Vileplume thing can be said for any Vileplume deck. That's one of the things that makes Vileplume a risky play.
I got "stalled" out plenty in Regionals at top cut. I almost lost in top two because he played so slow. It is COMPLETELY legal to take 15 seconds for every single card I play. I can also take 15 seconds to shuffle, etc. That really adds up when all you can do to me is 40 damage. I generally go to one prize against the deck, so no, you don't win on prizes. The same can be said for Swiss. If I take 3-5 minute turns, you're not going to catch up on prizes before time is called. Most Senior players don't do this. I only do it against these flip decks, because I find them somewhat unethical; more unethical than playing slowly. If I lose to them, it's because they flipped well.
It must be called Mewscoops because you only do two things every game: flip over Mew, and then scoop.