Firstly, we're talking about the Stronghold versus the Dungeon. If the opponent has already started with these heroes, how will you hire them?

Besides, I doubt anyone would argue that the Stronghold doesn't hinder magic development: Grand Magister is limited to level 3, Valhalla only gives +1 Strength, etc.

Secondly, in my opinion, the Dungeon's mages have better stats compared to the barbarian mages.

As for me, I doubt that the level 3 Grand Magister restriction on Might is often critical. But I understand your point.
We can argue about the heroes (I rarely see a start with both Jedidiah and Alamar at the same time, for the reason of "just so the opponent doesn't get them"; if the opponent hires Dimar on Might and spends money on whirlpools, universities, and other goodies instead of building troops, then the barbarians might be perfectly fine with a beautiful mage from a combination, for example, with the "Magic Arrow" specialization, the main thing is to arrive early; besides, you can't start with all mages - there are also Necropolis mages with the same Starfall, and towers with Chain Lightning)...
But this probably won't be a very constructive discussion... It's easier to just take a different city on Might...