So UI only kicks in when Goku's about to die.
I bet he'll never do it at will. This is just an excuse to let Goku fight in weaker forms, without hurting his "I win" button.
But it also means you know his life is never really on the line. If anyone could beat him down enough to beat him, UI kicks in and GG. Only way around that is a lot more Jiren level characters, which brings back the problem they were trying to avoid (An upgrade being degraded..)