Metagame role of PoL compared to alignment

[I'm A Banana]

I'm not trying to be too swift, but hasn't a defence of alignment's role in D&D painted itself into a corner once it's describing lower planar creatures, and their troubled interaction with the alignment system, as corner cases?

The only less corner case I could think of would be a Paladin.

You're not incorrect. Paladins and celestials and demons and devils are odd cases where the alignment is determined before any personality is applied. This is the exact opposite of how alignment was SUPPOSED to work in 3e (as a descriptor, not a proscriber), so it wound up making these creatures odd corner cases where their personalities might not entirely mesh with the alignment they were supposed to have.

This is more of an issue with legacy creatures and with poor writing than with alignment in my view, however, because the rules weren't always well applied, especially when things were carried over from earlier editions when the rules were different, but weren't changed because of 3e's desire to keep things as they were.