First you ignored the huge modifier in to hit modifier, and now you're trivializing the rest.
A difference of +3 to hit is hardly the end of the world. It will turn a hit into a miss on 15% of attacks, or maybe three times over the course of an entire day, for the wizards. And those are the crossbow attacks, which are a smaller damage contribution than your one huge spell, regardless.
"Slightly reduced defenses" - so are you giving them medium armor proficiency or making mage armor much better?
Because Mage Armor +1 Dex = 14, which is not slightly reduced. Especially when you're looking d6 vs. d10 for HPs and HDs.
Back-row combatants receive fewer attacks in the first place, so comparing Mage Armor +1 to splint armor, a difference of three AC is unlikely to come up more than twice per day. Such small differences only matter for checks that happen very frequently.
Likewise, the difference between 4hp and 6hp is only two points per level, before Constitution is factored in. Given that maximum HP doesn't affect healing from spells or potions, and everyone has abundant free healing with Hit Dice anyway, it's really not that big of a deal.
To get up to a +2 DEX, assuming you aren't forcing all casters to play +DEX races, is 7 of your point buy. That comes from somewhere. CON perhaps, but that's just a trade off that doesn't help their survivability.
This is 5E. Every race is a +DEX race, unless you're deliberately playing against type.
Humans are a +DEX race in 5E.
With Cantrip damage doing a lot less then weapon wielder damage, this is what the current system does. Yet that is not "more magical" according to you.
I'm saying that magic feels more magical when it only shows up in big bursts, rather than trickling out all over everything until it's indistinct and meaningless. The idea that cantrips don't "feel magical" was the premise of this thread, and I agree with it. So given that cantrips don't feel magical anyway, you lose nothing by changing Firebolts into crossbow bolts, except that the magic remaining in the system would appear more dramatic - more magical - by contrast.
Historically, it worked so poorly that they took that out and replaced it. The started bringin in at-will it in 3.5, didn't have it at all in 4e and 5e. It was intentional design decision to move to at-will magic.
We know that this game isn't perfect, or else we wouldn't be on this forum, discussing its faults. The fact that it's part of this game, or that the choice was deliberate, does not necessarily mean that it was the correct decision. After all, 4E was a deliberate decision, and we all know how well that turned out.