It would be nice if you could bribe powers to stay, make Free Cities, Imperial Favor, etc.
Say, if you can get them/keep them above 150. But if they go below zero, they can double-think about staying.
That way, a Lucca, who might be dependent on the Emperor, might stay, but a Milan that rivals Austria might leave.
I just loathe automagical and binary decision systems that are not really decisions.
Putting the huge maluses on the Italian states if they decide to stay makes it a fait accompli and a false choice. ("Gee, Mister Emperor, I'd love to stay and all, but look at the time... gotta run!")