Both of these depend on the state of the board. E.g. a Bish/Fort deck is the maximum VP possible on a board without any net draw. And if the Bish/Fort player sees any tricksy engine building, they can just mill a Bish or three from the pile and still make more than any possible setup.
The Bish/Fort player can't just mill a Bish or three from the pile without temporarily breaking the golden deck. Which is not a huge problem, but it slows down that process since you have to spend a turn trashing a Bishop every time your golden deck gets broken, unless you have good draws.
Beating 50 VP/turn is quite doable. The simplest shot being Possession. Even absent truly bonkers stuff like Masq or Amb, it takes exactly one play per turn to nuke their deck and score 50VP for you should they be so foolish as to leave you the opening.
I suppose Possession is a significant pain in the ASS if you haven't banned it and get matched against someone who also hasn't banned it, which probably isn't too often. You can't score the VP, however, you can only nuke the Horses.
Attacks also allow you leeway. Afterall, absent trashing, Stampede slowly dies once you start junking them. A Stampede hand only draws 15 total and if you dump the curse pile they will fall apart. Militias will at least drop the VP gain/turn. Things like Barbarian, Swindler, and Bandit can shut the whole works down permanently.
Trashing is not absent, it's not a golden deck without trashing unless you open 5/2 and never buy anything besides Collections and Stampedes (you don't technically
have to open 5/2 to do that, but opening nothing/nothing is almost certainly a bad idea). The fact that you have to play your trasher and therefore can only play 4 Collections that turn could be annoying, but it's a temporary annoyance at worst.
Militias will not drop the VP gain/turn, they just force the golden deck to be a bit thinner than it otherwise would have to be.
Barbarian and Swindler can eventually deplete the Horse pile and therefore get the game to end (probably still in the Collection/Stampede player's favor, but that's fine, at least the game will end), but Bandit probably just discards two Horses, unless you can play at least three of them per turn, which is a lot of Gold to deal with.
And getting into edge-case territory, there are many ways to score well over 50 points per turn. Collection/Villa/Butterfly/Seaway/cost reduction is unbounded points per turn (and needs only a single copy of everything except the cost reduction). Treasurer/Bishop can build out to 5 VP per combo with Plats which can go above 50 VP with things like Kc. Cost reduction/Grand Castle/trash diving (e.g. Graverobber), and gain 20 VP per trash & gain per cycle (e.g. Inheritance can easily put 20 VP cards into play/hand). And Kc/Treasurer/Cache/Tomb/Donate clocks in at 27 VP/turn without draw/+buys.
I'm not very confident any of those will be present in a typical Collection/Stampede game, especially with Stampede (which is always present in a typical Collection/Stampede game, surprisingly enough) taking up half the landscape slots on its own.
And I have already won a game by going Paddock/Collection instead of Stampede/Collection. Splitting the Collections results in 100 VP per turn if they don't contest the Paddocks at all.
Sure, but you need a way to play the Collections before the Paddocks, which is not usually present, and neither is Paddock.
The problem with Collection/Stampede is just that the combo is easy, obvious, and quick. There are many, many setups out there where you can generate truly insane amounts of VP repeatedly for turns. And many of those do not reward further engine building (e.g. at some point your deck caps out for Grand castle cycling). We don't care because they show up in something like 1/10,000 games once we account for alternative enablers.
2-card combos show up a lot more than 1/10k games. Collection/Stampede also needs a trasher, but that's pretty often there. I have already had a Collection/Stampede kingdom and I can't have played more than like a hundred games with Prosperity 2E yet, even that's probably an overestimation. Sure, people get it disproportionately often right now since you can automatch for extra 2E, but even under normal circumstances, 2-card combos happen. I just had a Hermit/Market Square game not too long ago, and that was while automatching for extra 2E.
The difference with Bishop/Fort and Stamped/Collection is just one of degree. Many times there will be nothing that can beat the quick and dirty golden run, sometimes there will be something that can. While I grant there are many more times where Stampede/Collection is untouchable, I doubt that there are all that many more boards where the other cards could beat Bish/Fortress all that often and if the raw point total is the concern, Paddock/Collection is a far worse offender.
It's also one of kind. Bishop/Fortress can do other things in addition to the Bishop/Fortressing, Stampede/Collection can't. Even if the "other things" doesn't involve building up to a bigger turn, it can e.g. involve a 3-pile ending with Fortresses and two other cheap cantrips which means you only have to be one full turn ahead of your opponent for the game to end.
The raw point total is not the concern, the concern is the stalemate. I don't see any reason why Paddock/Collection would lead to stalemates particularly often, since you can do it while you build an engine that buys VP and that will beat your opponent who is just playing Paddocks and Collections and whatever the thing is that lets them play the Paddocks after they have already played their Collections. I can see why Bishop/Fortress could theoretically lead to stalemates, but while I do remember people talking about this theoretical possibility a lot, I do not remember it ever happening to anyone in practice because the golden deck is far from unbeatable, and even in a mirror match, the player who gets ahead can almost always end it somehow.
I just did a quick search, skimming over posts that didn't look like reports, and found like a dozen game reports where a stalemate didn't happen, of which only a minority were golden deck mirrors at all, and the closest thing to a stalemate I found was
this which was probably not a stalemate either because it doesn't mention being one, but it also doesn't say how it ended so I guess it could have been one. Not a very scientifically rigorous result obviously, but I would nonetheless expect stalemate reports to show up a lot more than they seem to if that was a thing that actually happened to a substantial degree.
You could argue that I'm just being theoretical about Collection/Stampede stalemates too and that it won't turn out to be a thing that happens in practice either, and I suppose that is true so far, but a lot of the reasons why Bishop/Fortress stalemates don't happen don't apply to Collection/Stampede or require the player who's ahead to be ahead by a lot more in order to happen.