I don't understand your analysis on this comparing to Dex.
So the tradeoff is +1 to Con Saves and +1 to Con checks and +1 to hit points, vs +0 to Con saves and +0 to Con checks and +2 to hit points. Seems like a fair trade to me. Definitely not a no-brainer, and also not comparable to +4 to an ability.
If you're a caster, every monster in the game can force a Con save on a hit if you're maintaining a spell.
Simple. Some players do not want to waste time boosting Con since it mainly helps for hit points. So instead of boosting Con, they boost Dex and get a much bigger bang for their buck.
The more I think about it, +1 per level is not enough. But +2 per level, although not a no brainer, is a very strong candidate for a feat for nearly any class/race.
Let's look at the dwarf a moment. He gets a Con bonus so he gets more HP and a better Concentration check. He doesn't need Dex because he doesn't have to worry about Mage Armour do to his armour and I would say Initiative isn't that important. The strength bonus gives him morr carrying capacity. He also doesn't have to use up a spell slot everyday for Mage Armour so he is free to use it for something else. The dwarf gets to spend the rest of his levels focusing on Int and or Con.Dwarves, not having an Int bonus, do not net an additional spell slot for not using mage armor until very high levels when every wizard can be assumed to have an Int of 20.
By that point, you have so many spells that an extra one is not that big a deal. The elf's extra cantrip and sleep/charm resistance more than balance it out.
Another point worth making is that very few encounters should leave the wizard with no opportunity to get behind cover. Unless your campaign takes place in a featureless desert, there should be something to hide behind most of the time, granting full cover. With the relaxed movement rules in 5e, this means a smart wizard can easily make sure he's almost never a target of ranged attacks. Enemies with initiative might ready actions hoping he'll poke his head out, but if they do that, the wizard can decide not to oblige them (and the enemy attack is effectively nullified for the round, taking some of the heat off of the front line).
Now lets take the High Elf for example. He must decide which stats he is going to improve: Con to get more HP and Concentration, Dex to increase his AC since he doesn't wear armour, or Int to better his apellcasting. He can work on all three but he won't have much at higher levels.
I just think it's a mistake to write off AC, HP, and Concentration like some people have been doing. This is not the heroic edition where you have higher amounts of HP and spell casting has returned to being fragile. We also don't have all these immediate interrupts like 4th edition had so we are looking at a more dangerous edition.What the high elf may do is spend the couple of feats to get them to medium armour over the course of the first 12 levels. Those two feats may also net them a +2 to Dex so they could get to 14 with a starting roll of 10. What we don't know is if this also grants shield proficiency. If it does then they could be easily reach AC 19. Of course at the same time the dwarf could be pushing Int
However I don't imagine many players will want their wizards wearing armour (dwarves excepted) and I don't think many will be that bothered by the lower AC. We can't assume anything about magic items but I imagine bracers of defence will appear at some point
The more I think about it, +1 per level is not enough. But +2 per level, although not a no brainer, is a very strong candidate for a feat for nearly any class/race.
Yes the elf would need to spend two feats on medium armour proficiency and then need to decide if he wants Int or Dex or take a hit to go up in both. The dwarf's medium armour proficiency is roughly the equivalent of two feats and he gets the bonus to Con. He essentially gets what he needs just from his race alone. The high elf gets none of those so he is having to spend all his non racial resources on what he needs.
No, I understand why people want Dex. What I don't understand is why you want to mix the issues inherent with the Dex argument (that it is too powerful in 5e in general, compared to a whole host of things) with this issue. It's nonsense. You're conflating "Dex is overpowered" with "this feat is underpowered because in an alternate universe you might have instead chosen Dex which is overpowered."
So let's focus on the issue at hand, not the Dex issue. It's a canard. The obvious comparison is to increased Con. If you want to talk about the Dex problem with the game, do it in some thread about that topic. Because you could compare the Dex problem to anything in the game. It's entirely non-unique to this feat topic.
So we're again back to you arguing that a 0.5 difference is obviously overpowered.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.