If we work under this assumption, then I’d expect all the living material out there to be of the same chemistry and origin - again unproven conjecture but I’ll be surprised if two life forms of completely different chemistries can compete on comparable levels in the same environment - one will probably always be more efficient than other due to fundamental limits on the chemistry of their constituent molecules; I feel like new life of any chemistry can only form in a sterile environment since pre existing life seems to find ways to harness all available energy in any environment quite soon. Thus if we have life of a particular form floating ready to contaminate any new planet, then it’s possible that life of no other form can form in that galactic neighbourhood!

So the Star Trek episode where they discover why all the species around the galaxy all have DNA based genetics is probably on the money if you ask me!