I actually kind of like the idea of sentiments that last until you try to change them. It's happened many times to me that I've not gotten along with someone, and I will thereafter avoid them until one or the other of us tries to reconcile (and I know I hold grudges for a long time but lol). I turned off sentiment decay in my game, because the way that behaves by default makes no sense to me, if that says anything xD
I think something like that would add some dynamism to Sim relationships, if the choices they make could affect things like that, and the Parenthood bits could change over time. I mean it is sort of silly in the first place to make it like "so and so were BFFs as kids so they're BFFs for the rest of their life," like that's not realistic either? I don't talk to either of my childhood BFFs anymore, even if they're my siblings, so I don't think that's something that should be permanent either.