I mostly agree, but I have one differing note. I'd take a push on Amonkhet, where the Gatewatch doesn't really stop bolas but doesn't get destroyed either, to get a loss somewhere like Ravnica where the Gatewatch is on the defensive. For the sake of stakes, I don't just want to see the Gatewatch die. Heck, you could say that I don't really want to see them die: I think it could be more powerful if at least most of them live to see the villain's triumph. I imagine the circumstance where they go into Amonkhet guns blazing and get reduced to pasty smears by Bolas, but then I imagine the scenario where they survive that and return to Ravnica, only for Vraska to get her shot in at Jace and whether by his death or not destroy the Living Guildpact. Only for ruin, madness, sorrow, and destruction to be brought to the people the Gatewatch was supposed to be taking care of from the get-go, for them to suffer an utter defeat on their 'home' plane. I want to see how they feel, knowing their efforts have failed. Watching powerless to stand before the oncoming storm as Bolas and his pawns operating in Ravnica don't even have to brute force the matter, just give it a little nudge and the guilds themselves will ensure that all the Gatewatch stood for will be torn down, the fate of their people set along the road to destruction. And in the darkest hour, when the Guild War rages all around them, the Gateless, the little people like all those others the Gatewatch has purported to stand for, look up and they don't see oh big bad Vraska and bigger bad Bolas. They see the Gatewatch who brought this wrath down upon them, the impotent anger of the dying rounding upon the fools who dreamed themselves the saviors of worlds. And as for the Last of the Elder Dragons, the god-emperor of Madara? If he shows up, and flexes his muscle -- when he devours Niv-Mizzet and breaks Rakdos the Defiler to his will, when he claps his wings over the battlefield and the belligerents cease their conflict to gaze in terrified awe upon what has come before them? Who do you think Ravnica shall bow in prostration to now?

Of course, as I said, most of the Gatewatch members survive this scenario. Perhaps they even reform (With new "Get Bolas First" Tenets and/or maybe under Liliana's leadership on Dominaria), but they'll have that loss on their record forever, not because they bit off more than they could chew, not because there was an "accident", or a mistake and they 'walked into a situation unprepared'; not because some "tragedy" happened, or an unavoidable enemy action while they were away... no, they'll have lost because they fought their hardest, pushing themselves to the limits... and it wasn't good enough.

A good villain will manage a win where the heroes are weak and they are strong. A great villain -- someone playing on the level of, say, Princess Azula, or the level Bolas ought, can really distinct themselves by claiming a win where they ought to be weak, and the heroes strong, not just managing to foil being foiled and maintain the status quo, but really taking something away from the good guys that they can't really get back.