In general, stalemates come where one player cannot force an endgame. They will likely become more common as folks get better at Dominion.
I fail to see how the precise point total matters for the dynamics. Exactly which decks can best 30 VP/turn but not 50/turn? I mean, I could be wrong, but Ride/Collections beats the entire Colony pile in 3 turns while Stampede does it in 2. Even Supplies wins out in 4.
Sure, any golden deck gaining 20+ VP tokens per turn is usually unbeatable by a deck not gaining a similar amount of VP tokens. All the Collection/horse gainer golden decks mentioned will usually be dominant. Stampede is just the strongest of the group, as the higher VP gain means you need a much larger lede to be able to break the golden deck for a win, and the 5-card limit prevents you from incorporating it into an engine.
And I am not at all convinced that 2-card combos represent the majority of potential stalemates. After all, Kc/Monument is the only "2-card" combo (ignoring the whole, needs trashing thing), but Mastermind/Monument/Cantrip results in the same dynamic (high points/turn, potential game loss if you break the combo). As does Kc/Farmer's market/trash diver (6 VP/turn on average, unless you build out with Necromancer and then get 9 VP/turn). As do a lot of Bish/trash diver options.
I suppose with your method of choosing kingdoms, strong multi-card combos will come up much more often. But in pure random games, it's simple math that 2-card combos come up much more often than 3-card combos (by a factor of about 50 given the current size of the total card pool), let alone combos with even more cards.
Maybe I should clarify that by "n-card combo" I mean a combo requiring n exact cards, i.e. not counting common card types. So Mastermind/Monument/Cantrip would be a 2-card combo rather than a 3-card combo for me, as most random kingdoms contain a cantrip anyway.
Edit: Actually, Mastermind/Monument also gives a 9 VP/turn golden deck on its own, without needing any cantrip (only a "disappearing trasher" to get there, same as with KC):
You build a deck of 3 Monuments and 5 Masterminds. 3 Masterminds will be in play at the start of each of your turns: Two Masterminds are chained so that you can triple-play the 3 Monuments, plus a single Mastermind that triple-plays a new Mastermind from your hand. Then you play the last Mastermind from your hand with your free action to set up an identical next turn. The "completed" chain of 2 Masterminds/3 Monuments will be discarded during clean up, so you draw exactly those 5 cards for your next turn.

This golden deck might even be easier to set up than KC/Monument, as MM is cheaper than KC (and can thus be bought with the $ from a single MM'ed Monument).
Stampede/Collections is quick and easy to spot, but it is categorically not all that different from Collections/Ride or even Collections/Supplies.
I tend to agree; it is somewhat "worse" than the other two in terms of VPs and its engine incompatibility, but not on a completely different level.