Everyone gets a nerf because everyone can and does sometimes do more than just weapon damage depending upon things like sub-class, spell effects, items, etc. That includes fighters, monks, and barbarians, especially at higher levels. It's possible that the addition of inspiration balances it out for those few classes, but I doubt it. Rangers use hunter's mark as a staple, which would be nerfed. Etc.
Rogues aren't trickier. It's just huge nerf. Rogues don't need a bonus action to get sneak attack every round, they just need to engage an opponent who is also engaged with an ally. Not even that for swashbucklers. Rogues assume that they will be doing sneak attack damage on at least one attack per round and if they aren't, there are unusual circumstances or they don't know how to rogue.
Paladin nova damage is fun and class-defining. Hitting more reliably rather than seeing them occasionally light someone up makes for a more boring session, IMO. And the potential for nova damage is what makes them different from and helps them keep pace with barbarians and fighters, who already hit harder and more reliably.
It's not the average damage loss to Warlocks, which is minimal, it's the loss of the potential for a huge hit. In other words, the loss of potential fun. Rolling a 20 on your Eldritch Blast is a good time. Arguments amount the difference in average damage totally miss the point of what is fun about critical hits.
For monsters, a critting on a 20 makes it an event. Again, it's fun at the table, and raises the stakes. I'm not interested in Spelljammer monsters who won't see most tabletops, I'm interested in the Monster Manual and, to a lesser extent, Monsters of the Multiverse. This update needs to be backwards compatible, so even if wider damage variants might add more random threat to new monsters, it doesn't do anything for the vast majority that players will actually face: ogres, orcs, owlbears, etc. But I also don't think it's nearly as fun. And 5e is already a game where players can too easily mitigate risk, so it needs the chance of an unpredictable damage spike to keep at least some semblance of risk. Stories need stakes.
The current system works. A natural 20 on an attack is an event, and everyone gets it. That's fun. Something interesting happens. Natural 1s and natural 20s generate story. I am not interested in anything that makes combat, already the most boring part of the game, even more predictable.