The whole point of a "spending kill switch" is as a backstop when you make a mistake; but if you do it as a "DIY project", what prevents you from making a mistake on it? It has to be a built-in feature.
If its done first party, someone (realistically, af AWS’ scale, a substantial number of someones) will mess up using it and nuke their account.
Customer service can correct “we screwed up and have a giant bill” easier than “we screwed up and lost all our data”.
So its not going to happen first party.
(It’s also not really possible DIY as a hard certain cutoff at or before a limit, only at an indefinite interval in time and money beyond a limit. So you still have potentially unbounded expense.)
