There is a cumulative modifier, though it is sometimes overridden by hostility.A big issue seems to be that the AI will instantly break an alliance when you have one of their provinces of interest overlapping one of theirs. Why not use a cumulative modifier instead. Likewise with the rivalry system, why allow the trusted player country as a choice for AI countries when they have high trust with you?
If my next target is my ally, my next target is my ally. Alliances are something you use to get ahead of your competition (sometimes your allies, sometimes your mutual enemies, sometimes just a random guy) until you are strong enough to crush them, nothing more, nothing less. So the AI should break alliances if it desires land from you. Maybe it should be smarter about doing it, but it shouldn't stop altogether.
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