So you have a highly optimized group that broke the game and their interest was not in the character concepts themselves, but the mechanical superiority they have. That group not being interested in a Level Up is definitely a notch in the Positives column I think.
It’s one less group interested in the game, whatever your personal feelings on our play style. They are interested in having character depth, about half are pretty well as hard core on the role playing as roll playing, but I don’t begrudge any of them wanting to play interesting and effective characters and I build challenges to suit those alongside a story and NPCs to engage the role play heavy ones.
As a GM, it’s much more interesting to see fights against characters who make effective use of the system than vanilla builds that are as boring to watch as to play, honestly. I can craft encounters with powerful spellcasters knowing the abjurerer will tactical shut down one spell per round. I can send waves of strong enemies knowing the Barbarian/Fighter can probably lock one down with Polearm master and sentinel and the Cleric/Monk can do the same if one closes with Polearm Master and stunning strike. The pure fighter is straight damage but the group works to make the most of it, ensuring buffs like Improved Invisibility make the most of those action surges. The party can handle any sort of skill based test with good role play, critical thinking, and an MC build that leverages multiple expertises as well as the knowledge domain ability to learn any skill or tool, along with reliable talent.
I would always prefer to run for well built mechanically interesting characters than build around “softball” encounters, or non-combat challenges where the party neglected to cover anything beyond the ability to kill things.
Level Up seems to offer a whole lot of that. But it doesn’t offer it alongside the 5e builds my players prefer. If it did, I think they’d be sold on it, and so would I, as far as the player stuff. No one has issues with the monsters, though, which do make for more interesting fights from our first couple of uses of the new menagerie, so that’s something worthwhile.