Why is it important to have single-classed characters alongside gestalt?
Two reasons:
1) People want something that simulates actual MC'ing, as opposed to 3E's plate-stacking. Hybrid produced that, Gestalt kinda did but not really because it was pretty wackily OP.
2) Not all players want the complexity of Gestalt, which would be pretty high in 5E, relative to individual 5E classes (more than double, in some cases).
There's a choice. Either you can make the characters more powerful than a standard single-classed character and just have everyone do it and adjust your challenges to suit, or you can make sure the most powerful combo is no more powerful than a single-classed character and let a host of combos languish as weak-sauce "trap" options.
I find that the former choice is easier, speedier, and tons of fun. The negative is some added complexity and a bit of DM math, but if you're interested in gestalt, then you're interested in a more "advanced" game anyway (you're out of the realm of simple or even reasonably complicated character archetypes). This is not a very big negative, I think, and a large positive.
If everyone in the group is into it, that's lovely, but here's the thing, you barely even need rules for it, to do it the 3E way, do you? Seriously, what do you even really need rules on? Just take the features of both and where both have the same feature, take the better one. Done. No?
I find the latter involves more complexity, more analysis paralysis, and less satisfying results unless you're at the top of that power combo curve. The positive is that the DM can allow both at once and doesn't need to change her math at all, but challenge math is soft and squishy and entirely fungible by design already (since challenges vary even without gestalt), this just changes how big the numbers are at any given level. This is not a very large positive, I think, and a big negative.
I feel like you wildly overstate the negatives and difficulty here. I used several Hybrid PCs in 4E, and I didn't find them consistently under or over powered. They pretty much all seemed easy-to-handle and to work well with the other PCs. If they were "traps", too, half the classes in 5E are "traps", because the power disparities between individual classes in 5E are larger than between these guys and single-class ones.
Did you ever use the 4E Hybrid rules, btw, I mean, for actual PCs in actual games? I did, quite a bit. One of my favourite PCs was a Wizard/Bard.
It's reasonable to disagree on the intensity of those positives and negatives, and that might lead you to favor the latter over the former, but which one WotC decides to go with for 5e is likely going to be consistent with the 5e design philosophy, which favors playability over strict mathematical balance, so my wager would be on the former. Still, perhaps they found that the latter is a better approach!
I'd wager the former too, but not because of that design strategy, but rather because you could write up the whole thing in about a page, if that. That's why I'm saying it doesn't even really need rules.
One other option, I note, would be to strip the sub-classes off Gestalt characters. That would knock their power down a bit and limit their complexity and prevent probably all of the most broken interactions. Plus you'd presumably use rules like the MC rules for casting so that a Cleric/Wizard didn't have all the slots of both (because jesus wept...). If you did that, and say, gave single class characters maybe three free stat boost/Feat choices at L1, well, you wouldn't have parity, but you'd probably have something playable.
Ooooor you could strip the sub-classes, and DUN DUN DUN, bring in a DIFFERENT XP TABLE for Gestalt PCs, one which kept them 2-3 levels behind single-class PCs by L11, say, and we'd be basically rolling 2E-style multiclassing. HMMMM.
There's a lot you could do here.
I hope that if they only do one set of rules, it's for something more balanced than UA-style Gestalt (which as I will endlessly repeated, doesn't even really NEED rules-rules, it's so wacky), but maybe they can do even better.
If they can't and I haven't ceased caring about 5E by the time the DMG hits, maybe I can do better.