With regard to the beginning of this thread - the only way I can reconcile the concept of a carbon-silicone biochemical transition is something like this:

Some rare worlds have two biochemistries develop in parallel, rather than one type filling all available niches early in a world's history.

So the silicone-based life interacts sporadically with the carbon-based stuff e.g. silicone stuff is based underground, and quakes or volcanism allow contact with surface or ocean dwelling carbon-based life.

The real evolutionary action is evident at the level of micro-organisms - both in diversity and speed.

Multicellular life forms either develop symbiotic relationships, or die off from crippling autoimmune complications as a host tries to eject an intruder...