I agree, but I'm not too sure about that, but I also don't know enough chemistry to convincingly argue for the opposite..

Are there many varieties of self-replicating molecules? (which I understand is needed to start life) Maaaaybe there's only so many ways that this can get started and RNA is the "only" way to do it. In a similar way that we have a hard time imagining non-carbon life since no other element behaves in the ways that carbon does