I wonder if another filter for the proliferation of carbon-based lifeforms might simply be obsolescence with respect to mechanical lifeforms.

When it comes to space travel, machines have two huge advantages compared to carbon-based lifeforms:

1. They can persist indefinitely in space without much more than a solar sail to catch light energy.

2. They can evolve by intelligent design, rather than darwinian trial-and-error.

It seems likely that artificial general intelligence would tend to precede deep space immigration. And if that is the case, perhaps carbon-based societies tend to simply evolve into machines - especially those members of the species that actually want to participate in space travel. Space is not a pleasant place for a carbon-based lifeform. So those that want to leave the planet will become mechanical. Some of the species may remain carbon-based, but those will be the ones that remain on the home planet.

And once they are machines, they wouldn't need to endlessly duplicate, mutate, and die in order to evolve, as we need to do for darwinian evolution. And so they wouldn't need to spread like a virus in order to evolve and improve, like we are so used to life on earth doing. Instead, they could be content remaining small in number, since they can evolve simply by rebuilding or reprogramming themselves, rather than relying on continuous trial and error. The motivations for proliferation simply aren't what we as darwinian life forms are so used to.