Honestly I don't see the issue with cantrips at all. Most Wizards, for example, are going to want decent Dexterity, and a crossbow attack for d8+3 is better than firebolt in most circumstances.
Light, you say? Wizards don't use shields, so you could carry a torch, in dungeons you could even drop it on the ground when combat begins.
I've never encountered a problem with Mage Hand or Prestidigitation, and I think that covers the three most taken cantrips, lol.
Now obviously, the main point, for this thread, is that "well, those spells prevent you from having to worry about crossbow bolts or torches". But the canny mage can easily start with a bullseye lantern which is better than a torch anyways, and as long as they don't waste their oil trying to light random things on fire all the time the way my group did in AD&D, you shouldn't need to restock very often.
I actually recently thought I'd try holding onto flasks of alchemist's fire like the old days, but yikes! That stuff is expensive in the 5e PHB! 50 gp a flask? The heck with that, that cuts into the Spellbook budget!
My DM recently told me he was ok with me making scrolls, and I was like "uh....25 gp for a 1st level spell slot?!". He's lucky that the cost to scribe spells in my book is so much, or I might be tempted to destroy any semblance of game balance by buying a couple extra day's worth of spell slots, lol.
Anyways, at will magic is fine, IMO. You can argue about damage scaling, but by the time you're doing 2d10 or more with a firebolt, the threats have escalated significantly. Sure, you might take out a goblin with a cantrip. But you're fighting 20 of the buggers, lol, and they're smart enough to use bows.
(Another unsung source of ammunition and other resources, btw, the monsters need food, torches, water, and the like too!).
I'm glad you recognize the evils of the concentration mechanic, Lanefan, but I don't see your solution as being any more practical. I say no more than one duration spell running.
You say "oh no, have as many running as you want, but with a mechanic that can punish you for casting spells"- surely the end result is the same, where players are cautious and frugal about casting a lot of spells?
Then again, perhaps you simply have one of those groups that embraces the chaos and laughs whenever the wizard turns blue and erupts in a magical force explosion, lol. I mean someone besides me had to play Wild Mages in 2e!
I just found out the hard way that one should really clear it with their group before playing a Wild Mage. Or using a Wand of Wonder...or a Bag of Beans...or...
Well you get the idea. Not everyone thinks that sort of thing is "fun", so I wouldn't push it on anyone.