Don't all superheroes have a weakness though?
But, let's take a step back. Ars is a game about people. Some of these people have a great deal of power, but they are still people. Why do magi need to be able to withstand huge amounts of damage? What purpose, within the game, does that solve?
Champions killing damage is (or was, back in the day) very swingy; even a brick can be knocked out by one decent damage roll with a high STUN multiplier roll.
Levels are very useful. GMs can quickly stat up adversaries since a lot can be communicated and calibrated by "It's a Level X of Y." It gives players a solid idea of minimum and maximum power (skill levels, etc) appropriate for characters. And yes, level systems do not prevent players from "breaking" the system, but they more tractable than point-based systems in this regard. I don't think AM should go to a system with classes and levels, but both are useful and legitimate to consider even in a modern game design. (Heck, AM already is a class-based system! 
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It depends on the starting points as it is a point based system. And there are always trade offs for doing it. And it is all relative as the people you might be facing can also deal and take massive amount of damage.
To me it is about individualization vs standardization. Levels came from the original tactical studies from which TSR sprang, and they made units more uniform for calculating battle effects. Every level based system has moved towards a points based variant for greater customization. Switching from a character driven system to a levels driven system is a great way to kill a product line, and I can guarantee I wouldn't be playing it. The only advantage to a levels based system is that you can start the game quickly without having to develop characters because you aren't really playing characters anyways...
I think this is structural within ArM, because it's not a superhero game. Game mechanics come from a guiding philosophy, and ArM focuses in no small part on the mortality of humans in the Middle Ages.
Generally, a single sword blow can kill or maim an unprepared person, and that's on purpose. The miraculousness of magi are that they can easily transcend some of the limitations of mortal men, not that they are demigods out of the box.
Seriously: any game where the RaR requires spending 15 years to be able to have a basic shield against the supernatural and to hurl fire bolts only with extensive specialization is not a game that philosophically is designed for superhero-style play. Pick another set of rules if that's what you're looking for.
Not the same thing. Having a vulnerability is not the same as warding a vulnerable character.
I'm not sure that they need to be able to withstand huge amounts of damage. All I'm saying is that for other games, for high-action adventure games with ever-escalating stakes, for D&D - the class-level system is great.
The big advantage to having levels - or at least, high-resilience - in Ars Magica is that it would allow good defense against indirect combat spells. As a side-benefit, it would also make wizards control nature, rather than a "magical medium". I'm not sure these advantages are enough to offset the resulting ability of mages to shrug off the mundane's pitchforks. The only reason I'm wavering on this is that they can shrug them off anyway, with a little preparation. Still, I think the "they're still human" idea wins at the end.
Yes, I think that's a key part of ArM. A key part of why it's set up the way it does, and why levels won't work for it.
Levels are to make things even in a fight because that is all they are about. Ars and Champions do not have levels so they are about the characters and what they can or cannot do. I was looking more at the systems rather and the genres
If you want mages to have a higher resilience to damage without resorting to magic, there are HR for this. Exemple:
So, a magus with 5 in every form would have +3 soak, while his sodales with 10 would have +6 soak.
A Flambeau with corpus 5 and Ignem 15 could have +3 vs humans, +7 vs fire