It's also just like... gaming is different now and the culture is different. Games take themselves more seriously in ways they didn't use to in say the late nineties/early aughts. There was a whimsical air to early games where you know, it wasn't a big deal if you used cheat codes, you were just having fun by yourself at your PC. These days games are to a much further extent curated experiences that take themselves seriously and where it's important to get the full "artistic vision".

Also in the pre-internet times it was often a benefit to have secret stuff like cheat codes to generate buzz. I'm reusing an example from elsewhere in this thread but it was good word of mouth at the school/work place to have people spread the word about neat things like the Big Daddy car in Age of Empires or whatever. These days you could just find it with a quick search, and the magic and appeal of that kind of stuff is gone.

And again I want to pushback on the MTX angle a bit simply because we are still far from being at a stage where every game has them, even AAA games. Hell, if anything the GAAS and MTX wave seem to be abating. I would be much more on board with your argument if microtransactions were absolutely omnipresent, but we're not there at all, even in the AAA space. And I could absolutely see a retro-themed indie single player game add some old school cheat codes, I actually think at this point it would be a pretty solid marketing trick.

Regarding the achievements, there are many that only unlock if you are playing a certain difficulty or ruleset. It's not difficult to disable achievements while cheat codes are active. Plus people who are cheating with achievements (or high scores) often just fake it entirely and just send steam a fake score or achievement unlock, or use 3rd party tools/mods to do the cheating so the game can't even filter cheating vs legit achievements.

And this reminded me of a parallel to cheats offered by paradox games: custom rules. With CKII, you can create custom rulers without any limits. It'll say "don't go above this score if you want achievements, otherwise do what you want". You can even replace every single ruler in the game with basically mortal gods before you start and switch between them at will if you don't care about achievements, or replace everyone else with sickly imbeciles and dominate them all until they start dying off and getting replaced with normal characters.

And mods are another direction customization and replayability were taken vs using cheat codes.