I will just be blunt because I'm tired of MM's hogwash, its hogwash, pure and simple. Dismissed out of hand. The game is the game, and even if different GMs can play a given game differently that doesn't change the fact of the game itself having certain characteristics.
The system steps back 10yds and punts to the DM,
as part of its core resolution mechanic. Its written in 'natural language,' so is ambiguous enough to require constant interpretation & rulings, from the DM. It simply doesn't exist without one.
Is that hogwash? Maybe, but since MM washed the 5e hog, it's been immunized from criticism.
All I mean when I call 5e 'universal' is that the argument was effectively that it can be used as such, albeit with kitbashing (whereas GURPS required extensions for each genre, which is effectively the same thing, just supplied by the publisher and more cleanly organized as such).
I know from another thread that you've moved past the idea that the ideal RPG would be universal, but I still find it compelling. I classify systems as dedicated, core, multi-genre, or universal.
Dedicated is one game, one system, designed from the ground up, for that system. Obviously, the first RPGs were designed more or less that way, even if it was by degrees.
But, if you go to do a second game, and figure, why re-invent Oldowan Tools, you can strip away whatever doesn't work and build from there. Publish the stripped down version, and you've articulated a 'core system,' that you can use to build many games. AFAIK, Chaosium was the first to do that with Basic Role Playing in the early 80s.
Take that a little further, make each game built from your system mechanically compatible, and you have a multi-genre system. Like GURPS finally settled for being.
Get to the point that putting out a new genre book doesn't require new mechanics, and you've arrived at a Universal System. Maybe that even really was the Holy Grail of game design, though, it's sure not the Holy Grail of sales, your fans buy one weighty tome, and they're done until the next edition. ;P
So, 5e's sure not universal. Its got an SRD, so it's a d20 game. d20, itself, of course is a core system, and an open source one, which was an innovation - when FUDGE did it.
Frankly I think GURPS fails for the same reason that 5e can't be kitbashed into an equivalent of FATE (at least not in any reasonable way such that there is still some semblance of 5e there). And yes, FATE/FUDGE is a pretty universal STORY NOW genre independent system. It cannot do 5e any better than 5e can do it though.
I feel inclined to credit FATE as multi-genre, but I'm not sure various FATE games are all as cross-compatible as GURPS.
In any case, the idea of 'doing 5e' or 'doing FATE' is going pretty far down a rabbit hole, IMHO. Its not about doing a character in a genre having adventures in a setting, it's about apeing system artifacts. In FATE, Aspects describe all sorts of things about your character, in 5e, traits are tightly limited, but Background, Class, &c touch on some of the same things as Aspects, also - it is, I suppose ironically, a more complicated system, that way - 'doing Aspects in 5e' is like 'doing classes in FATE,' it's kitbashing to fake how the other system does something the native system already does, just with different mechanics.
Ironically, a Universal System could do that cold, without kitbashing... there's just no good reason to do so.
This is of course what TSR QUICKLY discovered! Metamorphosis Alpha/Gamma World was modestly successful. They didn't even TRY to do Wild West etc. as D&D clonesNote how Top Secret, Marvel, Star Frontiers, etc.
Boot Hill & Top Secret used similar systems, FASE-RIP was used in a few games, too, but they were never articulated as core systems.
all assiduously avoided any resemblance to D&D mechanics! In fact, after Gamma World 1e TSR never again released a game with anything like D&D mechanics, except D&D. In fact they tended to use the Marvel system, or a hack of it, as something like a generic system.
The 2nd and 4th eds of GW were also both D&D-like.
TSR also produced the Amazing Engine multi-genre system and used it for Metamorphosis Alpha, and, of course, Alternity, which afflicted the 5th ed of GW. Both to little success.