There is a clear separation between 1)consensus and 2)policy. The latter being each client's decision - this has at least been the argument of Lopp & co on why they can bypass "broad consensus ™️" in the ecosystem.

Let's entertain that thought. It works perfectly well in Lightning land: multiple implementations, adhering to the same base protocol rules ("consensus"), but differ in certain features ("policy").

Why is that/or not a model in Bitcoin mainchain land?