Fair points, I was more thinking of the mid to late game, when the world is pretty much divided between humans (depending on your game maybe 1600+). In the early game, it's not viable to flip so many provinces, since they are a large share of your land base. I'm also (unspokenly) assuming you are not playing with randoms who are out to troll you. Like I said, there could always be a betrayal, but would you really backstab your best buddy, larges neighbour *and* forsake a very beneficial agreement for generating Revanchism?
Depending on the final numbers, this is just a simple math problem. If you flip provinces that give you 15,000 manpower, you'd get 1,500 per year or 22,500 in 15 years. If your base manpower is something like 120,000, ie 12,000 per year then a +50% recovery decaying linearly over 20 years gives you 58,500 over the course of just the first 15 years. Hell yeah, I'd take that! If the tax increase is on the same level, it is actually beneficial to not keep the provinces but to give them back. There is no trust needed, just self-interest! And I can't even calculate the morale bonus because we have no numbers, but it will be very very potent since it directly increases your combat strength relative to your enemies, which is otherwise pretty much out of your influence apart from taking certain ideas and keeping your prestige high. And Revanchism goes on top of perfect combat stats too, giving you the absolute edge against the whole world!
