Erzskalde (Warchanter) / Erzassassin (just passing through - ignore me) / Erzsoldat (waiting for TR-time) / Erzschmied (ranged Artificer)
Yeah, Shame on you for asking a lore question on a forum and not immediately being silenced when someone on the internet says they are right.
He indicated he wasn't trying to institute a change, he said he didn't think lorewise it made sense. And hes been mostly non-argumentative, simply unconvinced.
Now, to the question. Adventuring doesn't imply toughness necessarily. He won't be weak by any means but given that his offensive power comes from within I suspect his ever-growing reserves of power come from the "exercise" of adventuring in some degree. I mean they explain it as innate power but it also makes sense to say that because he's running around using his skills constantly that his capacity to use them would improve.
As for being tough, given that casters in traditional play don't tend to stand and take punishment if they can help it, theres no reason to believe they would in the wild either. A sorcerer will still be a nuke and run sort of individual and thus will not have the same tolerance for violence as fighters and clerics that emphasize melee in some respects.
Further the Wizard spends his time studying and practicing, which means he takes enough hits of his own(from failed casts or the effects of new spells) to keep pace with a sorcerer.
Just 2 cents on why it might work.
I saw the question about why 'the healers' are running around in heavy armor, and had to chime in. In D&D, cleric does not mean healer. That's mainly a thing from video games, especially MMOs. D&D clerics are militant traveling clergy. They fight for the interests of the church and their god and their people. Sure, they can cure, they can heal, they can even raise the dead eventually. But they also call down smouldering wrath from the sky, rend the ground asunder with divine displeasure, call forth their deity's vassals to fight at their side (or for evil clerics, to serve them). They train to use their armor well, and how to wield their weapons competently, but their religious studies interfere in them getting the kind of mastery more martial classes have.
Paladins...paladins are the chosen champions of a faith. In the books, they don't choose to become paladins, they /are chosen/. Their powers come from divine favor, and can be taken away like nothing should they displease their deity. They train extensively in combat (granting them full BAB, martial weapon prof, and d10 hp) but not to the level of mastery of a fighter. Instead, they learn to channel their blessings into their battles, bearing divine wrath in their blade (smite evil), infusing it with holy power (bless weapon and holy sword), shattering the magics of wizards and clerics of rival faiths thought their god's power (break enchantment and dispel magic).
Both classes possess healing power, but it's because of their divine link. The reason those heals are Conjuration is because they are effectively drawing out a portion of their god's life force. Giving how much there is, and the rate of regeneration, it's pretty much unnoticable to the god, but they're quite capable of revoking it at a moment's notice, and stripping the divine caster of all their magic. A wizard is capable of healing wounds too, but the school is Necromancy, and the life force has to come from somewhere...either the wizard themselves (The name of that spell is Blood Bridge, where the wizard transfers wounds from the target to themselves) or a 'donor'. It's also possible to use a Transmutation effect to stabilize someone, altering their flesh and bones to prevent futher blood loss, but it's no comparison to divine healing.
Druidic magic is special in that regard, as it draws upon the ambient power of nature, and magnifies the body's own healing ability to levels that would make a troll feel silly and lame. It's the only form of magical healing where the energy doesn't come from some entity.
And yes, in theory, a wizard could make a deal with some powerful outsider to conjure away their life force as healing spells. But it'd be expensive to convince them, and the outsiders that are powerful enough for that...it's much better to just conjure the whole outsider, so they can use their SLA heals at no cost...unless, of course, you're masquerading as a cleric for some reason.
You make fair arguments. WotC thinks pretty much like you, indeed in D&D 5 something has changed.
For example, cleric now gets only light and medium armor proficiency, not heavy. On the other hand, sorcerers and wizards now have d6, while cleric still has d8.
Furthermore, sorcerers and wizards can use simple weapons properly, as you would expect. No more half bab compared to martial classes.
In d&d 5 many things are how they should be. You can easily think about a toon and then build it accordingly.