I think the basic problem can be summed up thusly:
Easy ways to shut down significant features of solos are bad.
I agree you can't just remove individual cantrip from the game. Blanket immunity against whole categories of features are also bad because it's impossible to fully investigate the consequences.
Look, even if Chill Touch is "just" a cantrip, when a level 17 caster uses it, it's not "just" a cantrip any longer. Now it's a feature that costs the full action of a level 17 caster! That is, the opportunity cost of this action is "could've cast Wish but didn't".
To me, this means that if a Wizard 17 spends his whole action on shutting down regeneration he should have a very reasonable chance of doing so. The regen is after all only shut down for a single round. And you do need to hit the monster's AC (though admittedly that won't be too difficult).
So if Tiamat or Juiblex is advancing upon the party amidst its most trusted half dozen guards and consorts, fine. Chill Touch does great damage (4d8 plus whatever the regen rate is). But the Wizard would probably have made great damage anyway.
And if the other monsters also have regen, theirs aren't turned off.
But when the monster is solo, everything changes.
Then the group has five actions to Juiblex' one. Suddenly spending one of them on keeping regeneration out of the picture isn't so expensive anymore. It creates a new fight, where the group has four actions to Juiblex' one - but now his entire side doesn't have regeneration at all any longer!
That's an curious way of looking at it, if you don't mind my saying so. I mean, if Jubilex's entourage doesn't have regeneration and you suppress Jubilex's regeneration,
his entire side no longer has regeneration, but so what? It's 20 hp that his side doesn't regain, big whoop. At the levels which you're likely to face Jubilex at, that's probably about one attack from the Battlemaster. You could have accomplished more with Fireball or Meteor Swarm, both of which
guarantee suppression of his regeneration, unlike Chill Touch which does nothing on a miss.
Look at it this way. There are 350+ creatures in the Monster Manual, by page count alone. By a quick count (I might have missed something) there are 12 creatures with regeneration in the MM. Of those, almost half are Slaad, which are by no means guaranteed to appear in every campaign unless the DM really has a thing for chaotic neutral toad dudes. So taking the remaining 7, less than 0.02% of the monsters in the MM have regeneration.
Even if we accept on face value that Chill Touch is OP against regenerating opponents, a spell that is OP against 0.02% of monsters isn't all that OP. Even if we include Slaad and all creatures with at least one healing ability, I'd guess that we're still dealing with less than 1% of the MM.
IMO, whether or not you feel that suppressing HP recovery is too powerful for a cantrip, Chill Touch simply isn't that powerful in perspective. I suppose if the DM went out of his way to homebrew lots of regenerating monsters I might be inclined to change my opinion, if only with respect to that particular campaign.
Having played a wizard in the Tiamat encounter, feeling completely useless other than casting a fly spell wasn't fun. I had exactly two spell slots to use against her. A 7th and 8th level slot. She would have likely saved or used Legendary Resistance to render those nearly ineffective. It's not fun for the player to be in that situation. It never was, not even when you ran into 90% plus magic resistant creatures back in 1st edition. It's not fun in a game with less spell slots. I doubt I'll use pure magic immunity as an ability for enemies again seeing the effect it had when combined with Legendary Resistance in that Tiamat fight.
While I freely admit that the capstone boss fight might not be the best place for it, I actually like those types of fights in general. I find it enjoyable to have to come up with unconventional solutions, such as using spells to make indirect attacks, rather than just spamming Fireball.