Coverting is not that easy.
Low level stuff? Sure. Higher level stuff? Not so much. 5e had a large weight of creatures from the 1-5 CR range,
True, but that is why ACs don't climb fast. Seemed to me the game was set up so most critters are a legitimate threat for some time in modest sized groups. Sure the band of bandits don't matter so much after a few levels, but they can still plink the PCs while allied half ogres tear them up and having send the fireballs at said bandits to keep them from pincushioning soft targets creates tactical considerations.
Mated pairs of dragons with young should wreck shop. And a band of book standard fire giants should be a big meaty threat for a very long time (though space might be an issue since 5E went back to having giants be huge.
and high level NPCs are non-existant. Takes me about an hour to create a high level spellcaster.
How many high level casters do you plan for the players to fight. Those types generally are not common unless one frees / summons a bunch from the underworld. Also a handful of lower level casters and a couple mid power mages might be better foe group rather than a single higher level caster doomed to being outgunned in the action economy.
But i'll agree, CRing an NPC can be a bit fiddly because the system has a modest bit of precision.
Edit: There is a bunch of new NPC foes in the Princes of the Apocalypse. Many are heavily elemental themed, but still a decent mix.