Welcome to Game Theory 101. In a game, if you offer players a clearly sub-optimal choice, it's likely they'll just ignore it. It's virtually never that it's the right decision for a mage to engage in melee combat from a purely tactical standpoint, so why shouldn't we make our characters so they ignore that option?
Not only that, it seems weird to ask why are D&D maintainers are sending D&D down the road of ubiquitous magic, and when people respond that it's because it fits how we play our characters, to tell us that we're playing D&D wrong.
Yeah. With that example, I was really thinking about 3E/PF. Then again, if they're a legitimate threat, my wizards are still not wanting to go within dagger-length of them.
There's a danger here though of focusing only on one class. Ok, we don't want the wizard (and the sorcerer really) standing on the front line. Fair enough, that's not a wizard's niche. Never has been. I can totally get behind that.
But, what about the cleric and the druid? A cleric on the front line fits pretty darn well with its niche. It's SUPPOSED to be there. That's why it has the best AC in the game, almost the best HP and almost the best weapons. It's not as good as the fighter, true, but, it's no slouch either. But, with the shared spell lists and the fact that at wills replace attacks, other than a war priest, there's virtually no reason to stand on the front line. Max out your Wis score, dump stat Str and away you go. That's pretty far from what clerics used to be.
Clerics never used to be blasters. But, again that changed in 3e. Clerics got all sorts of area effect attack spells at pretty much every level. Granted, in 3e, the best option was still likely to buff the crap out of the cleric and go to town with your mace, so, at least the cleric was kinda close to his roots. But a 5e cleric? Pew pew away with Sacred Flame, Light Domain and I've got Burning Hands and Scorching Ray and Fireball. Tempest Domain and I've got Thunderwave and Call Lightning. I'm sure there's more.
There's no particular reason for my cleric to ever get into melee combat. ((Again, excepting War Domain clerics obviously)) And that's ignoring Domain powers, that's just spells.
Once upon a time, a lot of effects were siloed into specific classes. If you wanted an area of effect damaging spell, you needed a wizard (or a very high level druid or cleric). If you wanted to fly, teleport, or summon monsters, you needed a wizard. Now, druids are summoners, and clerics can make you fly.
I brought up summon monsters for a reason actually. I LOVE summoners. Always have. Played them all the way back to 2e and probably in 1e as well. I loved how you could call up a small army of minions to do your bidding. Tons of fun for me. Now, the only way i can actually play a summoner is play a Druid? Since when are druids summoners? But, I suppose that's a separate issue. But, what that has meant is that my druid is pretty much a conjurer in all but name. Virtually all the effects he uses, if they were in 3e, would be conjuration effects. By and large, with a couple of exceptions (invisibility being a big one

).
By making most of the spell lists very similar - everyone has area effects, everyone has mobility effects, everyone has effects that are similar to each other - and then opening up other classes spell lists to differing classes, 5e has made every caster pretty much the same thing. To me, this is the problem with ubiquitous magic. It's made all the classes play very similarly to each other.