The Magic pack: does it ruin the game for non-magic sims?

No as long as you don't step foot in the Magic realm!

The Sims that you have played or your Sims have interacted with won't be turned into Spellcasters, according to the SimGurus. Only unplayed townies will show up in the Realm and only when you go there with your played Sim.

So if you never visit the Realm, there's not going to be any magic in your save.
If you visit the Realm with a Sim, only Sims that aren't played by you or on your Sims' relationship list will show up and be Spellcasters.

Also, even if Sims turn into Spellcasters, you won't see them do anything magic outside of the magic realm other than fly off on a broom.

Someone created a mod that stops the game from turning other sims into spell casters.

https://www.patreon.com/posts/quick-mod-for-my-29866791

Since I'm wondering the same thing, I'm gonna bookmark this thread.

If you don't have any Spellcasters among the sims you play, don't go to Glimmerbrook, and especially don't go to the Realm of Magic - you will see no magic in your game.

I created a new save and played for about 5-6 Sim days staying in San Myshuno. I saw almost every Realm of Magic premade - but they didn't do anything magical. Didn't even pick up a broom.

I then went through and checked every household in household management. Only the Realm of Magic premades's were spellcasters. Vampires were more intrusive, and they were hardly intrusive. (I got a visit from Vlad and there was one Townie Vampire in addition to those from the Vampires pack)

I have saves where I have never visited the magic realm and aside from the pack items in the bb catalog and in CAS I would never know I had the pack installed. Even in the saves where I have visited the realm, I haven’t found muggles performing magic in the regular world outside of my own controlled sims.

I have two separate files. In one file my Spellcaster has never visited the Realm of Magic and besides them and the Spellcasters in Glimmerbrook that they invite over to duel I haven’t seen any of my townies doing magic. But on my other file that has a Spellcaster in a feud with another Spellcaster I created but don’t play, when I take the played Spellcaster to Glimmerbrook I have seen the Townies the game spawned to fill homes appear in the Realm of Magic and one regular Townie. In the Realm they fly around and duel autonomously, but aside they act like muggles.

But now I’m interested if my other Townies in any of the files are pretending to be muggles. Maybe make a club to see unless the game will just assign them the trait to fill the club.