Unfortunately, I'm ignorant on this topic, and my main source is: https://en.m.wikipedia.org/wiki/Particle_decay
If particles only decay into less massive particles, what keeps all the different massive particles around? What keeps the universe from rapidly devolving into pure electrons and massless bosons? What keeps non-electron fermions around? I must obviously be missing some very crucial details.
Your question seems to be:"Why haven't all the massive particles already decayed?"
The answer has two parts. The first is that some particles (e.g , free protons and neutrons in unstable nuclei) have very long decay half-lives. The second i s that new massive particles are continually being produced in stars and supernovae.
As a caveat, this answer will be only in reference to free (or relatively free) space at very low temperatures, as more things can happen in a medium or at higher temperatures.
Decays result from certain interactions, and using the formalism of quantum field theory, these interactions all conserve certain quantities. Conservation of energy and momentum is what is responsible for particles only decaying into less massive particles in free space, for example. So the reason that, say, protons don't decay into electrons is because this would violate conservation laws.
In particular, it would violate both electric charge conservation and something called baryon number conservation. The proton is the lightest baryon, and so there is no way it can decay without violating conservation. Similar statements hold for quarks and other kinds of particles. This is what makes certain particles stable: there aren't interactions that would make those decays possible.
(Additional caveat, you can always imagine adding new interactions, such as in grand unified theories, which as an example allow the proton to decay, but then it becomes a question of lifetime, and these particles can have lifetimes much longer than the current age of the universe).
