Take it from me, a stall player and heavy Quagsire user,QUagsire isn't niche. Quagsire can counter many pokemon in this metagame that are dangerous because of their set up such as lum DDnite, SD chomp and the biggest one: DD Zard X. It is a great defensive glue to many teams, often used in stall but it can be effective in other playstyles if used correctly. Quagsire is a great pokemon and is almost mandatory on current serious stall teams such as my very own
Goldfish or previously mentioned
TFL 's teh fgts that nevr die.
Unaware in general is a great check/counter to end of chain BP sweepers. I do think however that the real threatening thing is that offense is getting damage instead of stall unlike every other suspect and that is what causes such an upset. So far my stall team often beats baton pass due to unexpected roar on mons like Mega Venu and use of high packed defenses, and unaware.
I think that Mega Venusaur is one of the best pokemon to combat this playstyle along with Quagsire. Mega Venusaur laughs at BPs highest regarded win condition and can unexpectidely roar out things like scolipede. I tihnk the best thing to do when seenig BP is to lead with a roar user as even if they pivot into espeon they don't really have any boosts yet,
mold breaker taunt is another great way to break these chains. Here are some users of MB Taunt:
Basculin, Basculin-Blue-Striped, Druddigon, Fraxure,
Gyarados-Mega, Hawlucha, Haxorus, Pangoro,
Sawk(possibly, it's like C or + rank in OU but baton pass stopping and knock off give it a niche over things like megacham and terrak), Throh
In Bold are the most viable ones. I actually used tauntDD Gyarados in my older RMT Adventure melodyand it wans't only niche to beat BP, heck I barely faced BP on the alt, it was mainly to break through heavier walls from recovering, and it worked great.