Is extreme cladism a mainstream position in the species debate?
I've never heard of it, actually - though I can't speak for my professors or researchers I know, none have ever presented the Cladism argument as an argument for the delineation of species.

As far as I'm aware, the attempts to define a species fall along three lines:

The ability to produce fertile offspring in the wild.

Distinct physical characteristics.

An arbitrary amount of genetic divergence.