I suspect Tony and Banana are correct - that the devs intended parties with magic users not too different to standard dnd parties - it's just the rest of the world that is low magic. And.... it is there that I think the setting falls down really.
A high magic party in a low magic world is going to be... kinda OP?
Anyhoo, I still like PT a lot. And I understand the devs want to appeal to the big 5e crowd.
I don't get the sense that they were forced into this decision. I mean, PT was born out of a kickstarter, there's
zero reason to suspect that they didn't make exactly what they set out to make, and recognized the appeal of playing dealer in dark and forbidden arts as an appealing character choice (a la a standard D&D warlock). If they wanted to make something else, presumably, they would've kickstarted that other thing. That's one of the benefits of kickstarter: you don't HAVE TO make anything, and you can try weird things with a niche audience!
I also don't think that magic-use in a low magic setting is inherently powerful. For PT, I get the sense that if you refuse to play by the setting's fiction - magic is dangerous - than the DM can have that fiction fall right on top of you. While you can use cantrips as much as you want out in the dungeon to slay the lich or whatever, if you're casting
charm person on the local innkeep, your party is going to be run out of town with pitchforks, or you're going to have a campaign about slaughtering townsfolk.

It wouldn't be much different if an assassin came to town and started wearing a sign that says "I Kill Nobles": you're dangerous, you're unpredictable, you need to leave. Players do need to consent to that fiction (and those that don't probably wouldn't be big PT fans!)
But playing a caster in PT seems, at least from the default assumptions of the setting, like essentially telling the DM, "I want to play a character who trucks in dark forces. I want my power to be dangerous. I want to be feared. I want stories that involve the Great Old Ones." As a baseline, that seems to be an acceptable kind of character in a grim setting like PT - certainly you can be a heartless murderous mercenary who is dangerous to cross and who kills people at the drop of a hat and who maybe fights a squid monster from space at some point.
Rather than banning spellcasters, some house rules more in the PT mold as the setting is written might be a few variant flaws for your spellcaster character (like "I pursue knowledge no matter the cost" or "I hear whispers of bizarre creatures in my dreams, and they terrify me...but I don't want to be rid of them...") .
That is, if you're looking to flow in the mold set by the designers, and not just use PT as firewood for your awesome low-magic-setting bonfire. In that latter case, just ban classes with cantrips, and run with it!
