When you send your laptop to Apple, they also try this compressed air trick (instore at least in UK) - and if that fails, they do the replacement programme. There are two types of failures, one is indeed something merely being stuck underneath the key, the other is total key failure.
YMMV with what your store/Apple employee do of course. But it does beg the question, why should anyone have to use compressed air on keyboards, I mean to date have you ever had to prior to butterfly keys? I have used and seen keyboards with enough gunk beneath the keys to be a potential health hazard, still typing away perfectly fine without breaking.