Oh snap! Precedented!
You're completely missing the point.
If the only way in 5th edition to gain this ability is through the fighter chassis, that's how people will get it.
Eberron feat didn't have prerequisites, so anyone could take it. Then it's either unbalanced on its own, and can be banned easily, or it isn't an it isn't a problem. By my own playtesting experience, it can be a problem when you can offload 2 daily spells in rapid succession. If it isn't unbalancing, why didn't they give a similar ability to mages? Eldritch knights are going to be very popular in 5th edition, I guess.
I like multiclassing when there are lots of viable options. Not when there is only take this, because any other choice is super weak in comparison. I find it surprising that so many compain about classic wizard power due to their spells, yet fail to realize that having the ability to be twice as powerful when you can offload such a huge burst of energy than your opponents or other single classed mages, and suffer little drawback (Giving up your 20th level capstone isn't a very big penalty for having the ability to nova so spectacularly throughout the duration of the entire campaign), is much worse.
I actually don't think it's overpowered for fighters. I like that fighters can get something nice and be able to whoop some behind once in a while. But when the mage can swoop in and steal his best stuff for a 2 level dip, and benefit his main class better than probably any other MC combination, then I start to wonder what the devs were smoking when they let this slip through. Did they not have any powergamers playtesting this edition? I don't think I would ever play a mage who didn't multiclass just for this ability, even if he wasn't interested in wearing heavy armor or going into melee. Extra HP and saves are a nice bonus, but being able to nova any two of your spells from your ever increasing spell list? Yes, please! I'll take that. Each and every time.