number of lions in european zoos

Again, that depends on your definitions. As I’m sure you’re aware, three-dozen different species concepts have been proposed in the literature and, as I said in my previous post, the species classification is also an artificial one *to an extent*. However, I tend to follow Darwin on this point: every biologist agrees on what constitutes a species in most cases.

That’s not the case with subspecies where, as you point out, most fail to meet your criteria. Not to mention, those criteria are extremely broad. Most populations have a unique genetic signature ("differ in genetics"), but that doesn't make them different subspecies. As for the difference in "appearance", there's no clear distinction between what constitutes subspecies-level morphological differences (even species-level) and what doesn't. Phenotypic plasticity further complicates the issue, whilst preserving genetic diversity even without morphological divergence is important.

The BSC has many well-known flaws (especially outside higher verts), but is a good example of where defining a species is not artificial, whereas defining a subspecies is. I also prefer it to the PSC, which is fundamentally artificial. That is, the requisite degree of differentiation for populations to be considered separate species is necessarily arbitrary.

Hence why I tend to think in terms of ESUs: which populations should have their genetic integrity preserved (maintaining diversity at the species level) and which should we interbreed (increasing diversity at the individual level)? This is a practical definition, but relevant from a conservation standpoint.