The gist of how fermions transform seems to be that anything can become anything, as long as certain properties are conserved.
That would mean that it's easy peasy for a particle to lower in energy, all it needs to do is split into a combination of particles with the same summed up properties.
But it'd be far less likely for the transformation to go the other way, because it's not a spontaneous transformation; a low energy quark would have to receive a fucktonne of energy to become a top or bottom quark (assuming direct transformation between quark families is possible).
In short, having heavy fermions requires counteracting spontaneous transformation with lots of energy available to allow transformations in the other direction.

Am I right in this being why heavy fermions are rare?