I think they will already act like that, MIPS or not. Stick a helmet on your head, now wiggle it. It moves really quite a bit, unless you like to wear it so tight you get a headache.
Is it worth helmets that are 100g heavier and don't breathe as well, though?
I think that’s a false dichotomy.
My Lazer Genesis Helmet is a MIPs and it’s the lightest helmet Lazer made at the time.
Much more breathable than my previous helmets too.
You forgot to mention it's also $200+. Some folks buy bicycles for less than that.