That's a lot of time for Pathfinder to grow and do its thing. It was able to cement itself as a "classic" TTRPG experience while Wizards was dealing with 4E, and eventually trying to recover and turn things around with 5E. Given 4E's incredible unpopularity and poor reception, Pathfinder was incredibly well positioned to build its player base and name recognition as D&D floundered. Paizo was already publishing for 3.5 and had a lot of name recognition and popularity, so it was able to capitalize on that position with Pathfinder which "preserved" 3.5E as Wizards moved on to 4E. I'm not a roleplaying history expert, but my understanding is that Pathfinder's initial popularity resulted from D&D being in a really vulnerable place at the time, coupled with their uniquely strong position as well-known former D&D publishers. With that in mind, LU is kind of trying to follow in Paizo's footsteps. It was essentially just carrying forward a lightly-modified 3.5E (thanks to the OGL) as Wizards turned towards 4E, and was not very original (yet, at least). Let's clarify Pathfinder's origin: it very much was not a distinct d20 system. TL DR - What is it about 5E that is so good they made LU a derivative system of 5E, but so bad that they felt they had to make LU in the first place? But since they did, why did they feel the need to base it off of 5E instead of coming up with a distinct d20 system the way Paizo did with PF? What I want to know is, why did they do this? They could have just codified some of the variant rules that came out in TCoE and made heritages and backgrounds, and included drop-in rules for exploration and updated monster stat blocks without going to the trouble to make a completely new system.
Essentially it is its own game, using the same concepts and conceits 5E has as its base. But what I'm understanding so far is that LU is 5E with some PF2E concepts added in, and that it would be hard to pull in some elements of LU into 5E without just using LU. I came across Level Up: Advanced 5E yesterday, and I looked into it a bit, as well as reading this reddit review from u/MC_Pterodactyl and the Play to See What Happens blogpost on it.