(2) The classic D&D monsters with their baroque immunities and vulnerabilities are basically otiose in this sort of play. Those monsters were invented to challenge players, and to force them to learn different winning formulas, Over time (and perhaps over the course of multiple PCs), players would improve their knowledge of the game elements - trolls, ochre jellies, yellow mould, etc - and be able to defeat them more handily and thus do better at the game. (This is a point that
@hawkeyefan has also made.) Once play is no longer focused on challenging players, such that the notion of "unfair advantage" doesn't really apply, why do we bother having all these different trick monsters? They've outlived their usefulness.