Actually, yes, we do need more rules for multiclassing. You may or may not be familiar with the AD&D style multiclassing, but it allowed you to simultaneously advance in 2 (or 3--but that's probably overkill for 5e) classes at once. Those who never used it or aren't familiar with it generally don't realize how big of a deal it is. It's a whole different animal, kind of like 4e hybrid classes, but different. The 3e-inspired system does what it does alright, but it fails to address issues (primarily, higher level spell access) that those of us who are looking for another option want addressed.
I'm familiar with 1e multiclassing. I've been playing (actually, mostly DM'ing) since 1981. However, it didn't really let you simultaneously advance. Yes, your XP was split evenly between the two or three classes... but the classes each had different XP requirements to level. My elven fighter/magic-user/thief (i.e. fighter/wizard/rogue) needs 1,250 XP for level 2 thief, 2,000 XP for level 2 fighter, and 2,500 XP for level 2 magic-user. Later levels have an even bigger disparity, as XP requirements tended to double until "name level", which means that my thief levels are going to rapidly outpace my magic-user levels.
So, what does this mean? Well... as I gain XP, I'm going to "level up" in thief first. And then fighter. And then magic-user. In fact, I may get to a point where I'm leveling up twice as a thief before I level up once as a magic-user.
What I'm struggling with is how 5e is so dramatically different. Start out as an elf wizard with criminal background. Bam: you're a sword-wielding, longbow-toting spell-caster with proficiency in Stealth and Thieves tools. Pretty good for 1st level. Let's rock on.
You gain XP. You wish to multiclass. You take 2nd level in rogue. This is consistent with 1e. You're going to "level up" in thief (rogue) first. Take 3rd level in fighter. Take 4th level in wizard. Etc. Or, alternatively, stick with fighter and rogue levels, and subclass into eldritch knight or arcane trickster. 5e has improved the multiclassing rules, such that your proficiency bonus (and hence your "thieves skills" and "spell power / DCs") are always going up with total character level. This avoids the old 3e trap of having absolutely useless abilities from your secondary or tertiary classes at high levels.
"...but Lancelot, your approach means that I'll need obscene amounts of XP to get to 3rd level wizard!" Sure, but you needed them in 1e as well. At the same time your elven F/MU/T was hitting 2nd level in wizard (7,500 XP, split three ways), the party druid was already 4th or 5th level.
"...but Lancelot, your approach means that I'll never get high level spells!" You weren't getting them in 1e either, unless you were rockin' Unearthed Arcana and had basically removed the wizard level caps from the elves. Most non-humans were limited to mid-level fighter, mid-level spell-caster, and unlimited in rogue. With the exception of (say) Unearthed Arcana gray elves, you were never going to be casting the 9th level slots. That was for the human magic-users only. Also, with the 5e approach noted above, and assuming you eventually subclass into eldritch knight and/or arcane trickster, you could be seeing something like the following in 5e: Fighter 6 / Rogue 6 / Wizard 8. Based on the multiclassing table, you add a third of your arcane trickster and eldritch knight levels to your wizard level, for a total effective spellcasting level of 12th. That's good for 6th level spells, which is about as good as a 1e elf
ever got.
Given you can achieve a broadly comparable experience to 1e (as described above) with the current 5e rules, I still don't see the point in taking valuable space from the DMG for this extreme corner case. Worse yet, this is clearly an area where there is almost no codified response that is going to satisfy the minority who want some kind of gestalt 1e multiclassing variant. What can they conceivably do, that would be balanced?
Some kind of points-buy system, where you can "buy" skills or spells from the class of your choice, each time you level? That's not 1e multiclassing - that's 2e "Skill and Powers" splatbook stuff, and that was pretty widely reviled at the time as being a munchkin's delight.
Maybe break up each level of each class into halves or thirds, so you can take a half-level of fighter and a half-level of wizard each time you level up? That's incredibly finicky, and massively complicates the design of the game. That'd be dozens of pages of material, simply to describe how to do it for every class.
Maybe double or triple the XP requirements to level (much like 1e, which split your XP in half or thirds), and then you gain a level in each class when you hit the threshold? That's functionally similar (and clearly inferior) to the current 5e method.
I'd honestly be interested in hearing an alternate "gestalt / multi-classing" method from one of its proponents.