Are you honestly trying to suggest that Gastly is a shitty Pokemon?
Gastly is not shitty, but it's not quite a defensive Pokemon; many, many moderately attacks are able to cleanly OHKO Gastly through its atrocious defenses, not just Knock Off.
Or that Phantump is a shitty Pokemon?
Spinblocking isn't what it used to be, since Defog tends to be the far more popular method to remove hazards. In addition, being the best available check to Drillbur isn't actually a super notable feat, considering how Drillbur isn't even A+, let alone S, on the viability thread, and is prone to being beaten through a variety of other tactics, even if they don't block Rapid Spin. Its bad match-up against a lot of common pokes, including ones that don't carry Knock Off, such as Misdreavus and Fletchling, also hold it back. I'm not saying Phantump is a totally awful Pokemon, though.
Taken straight from the Misdreavus analysis: However, with XY came the rise of Knock Off. Misdreavus is OHKOed by Knock Off on less bulky sets and does not appreciate losing its Eviolite on the bulkier sets.
Despite its weakness to Knock Off, Misdreavus maintains its position as one of the most used Pokemon in LC. The point wasn't that Misdreavus didn't mind Knock Off, but rather the exact opposite; despite Misdreavus possessing such a weakness to Knock Off, it is still often considered the best Pokemon in LC, meaning Knock Off wasn't nearly enough to hold it back.
I don't think sturdy juice is the prob, but sturdy smash with berry juice certainly is.
hazards break sturdy, but he can still shell smash on whoever he switched in, and most probably you'll have to sacrifice at least 1 or 2 poke and then have a strong priority to kill them off 100% or you might risk proccing their berry juice and they're back to full and you basically lose.
Unless you have a check or counter to them, like Mienfoo, Sucker Punch Pawniard, Timburr, Chinchou, Cottonee, Croagunk, Magnemite, Porygon, and a few others
I don't see how this is that much different from swirlix case in which he could set up BD and unburden at first turn in many cases, and he didnt even had sturdy. The only major difference in fact is that swirlix could set up on the much common fighting types, and no one was really willing to run Poison jab or poison types specifically for that and stealth rock were much more uncommon. But now i see people running pjab and poison types alot specifically to deal with fighters or fairys
BD wasn't even Swirlix's most notable set, if I recall. There isn't really anything Tirtouga or Dwebble can pull off to match the CGCM set. Even then, what really put Swirlix over the edge was that it could run a multitude of completely viable sets that could each sweep teams that didn't have dedicated counters for them, and had different counters.
It's not like smashers don't centralize the game at all, in fact, one of the biggest reasons priority and stealth rock is so common is exactly to deal with them.
The most popular priority user is Fletchling, and it's not dealing with Tirtouga any time soon, so smashers probably have little to nothing to deal with the popularity of priority; priority is just really, really good in its design. Stealth Rock is definitely not common because of smashers alone, because its utility extends far beyond that; it has repeatedly been deemed in every singles tier since its release, including the gen 4 metas where Shell Smash didn't even exist, as the best move in competitive play.
One more thing - keep in mind that Tirtouga doesn't actually completely obliterate everything ever after just one turn of set-up lol, especially with Eviolite everywhere.