>But if photons couldn't decay due to not experiencing time, they couldn't do anything else either.

I mean we are jumping way out of the classical behavior that objects like you and I exist in. To the photon itself is a timeless object. It 'moves' in a null geodesic where t=0. Attempting to apply any classical behavior that occurs in time-like objects just isn't going to work when applying them to massless light-like objects.