Mh. I think the unconditional aspect would actually lead to a pretty neat dynamic.
Suppose your opponent is going for Cartels. They're not going to buy Provinces, which means that you need all eight of them to end the game. This is also the case if your opponent goes for alt VP, but the difference is that alt VP accumulate VP gradually, whereas this is all or nothing, which makes it so being 1 point ahead is as good as 25 Points. This means that you'll essentially always want to 3-pile; buying Provinces is likely to be a waste of money.
Conversely, suppose you are buying Cartels. In this case, you're just waiting for your opponent to start greening, so that they acquire useless victory points and handicap their ability to three-pile. This should make your opponent hesitant to buy the first Province, which means they'll build on their engine instead, which in turn makes Cartel worse because the 3-pile is closer. Given all that, optimal play should be to delay greening on both sides, estimate the point at which Cartel is no longer worth it, and then go green. Alternatively, buy Cartels if you think your opponent estimated incorrectly and bought the first province too early.
If Cartel did say "+X VP, end the game" instead, this would change the dynamic a lot since now you do want to buy Provinces against it. I'm not sure how it plays now, but it feels more similar to other Alt VPs, which I don't like.