I love how you're so confident, d master. Look, I see where you're getting at, but a deck that needs no energy to attack versus a deck that needs two energy attack. Sure, it gets the immunity, but after one 80 + 10 from poison, it is only 30 damage from getting KO'd. I'd like to mention that Mightyena decks, despite Mightyena having free retreat cost, run 2-3 Warp Points anyways, which could easily save them from this situation. Sure, Mightyena isn't that great, but I don't like how you're so confident that just because there is one card to counter it, it will fade out. Dialga G didn't fade out (or the G's) when Machamp showed up to kill it. Heck, even when Dialga G was out people played Mightyena.
ToxiTank, I doubt, will also fade out. First of all, if you start the first turn, you're fine, still. Unless your opponent has the ultimate perfect hand in which he can somehow get two energies onto Trapinch then Rare Candy into Flygon, you're not in bad shape. If you go second, you have the chance of donking. Sure, you can't always rely on donking, but you can't just eliminate a deck from the metagame just because there MIGHT be a counter that MIGHT see play in the metagame.