Where the Lower Tier Players Handbook covers tiers 1-4, 5-8, 9-12, then the Upper Tier Players Handbook can mention the variant option of jumping to the epic tier 21-24, instead of continuing on with tiers 13-16 and 17-20.
In 5e, the tiers 9-12 work fine. The gain of new caster spell slots is already slowing down.
Okay, gotta ask. Where you getting those levels? Because 5e is broken into four tiers 1 through 4, 5 to 10, 11 to 16, and 17+. Breaking up the established tiers is odd. Especially for warlock players.
Either way, still doesn't change that going to level 20 is very likely a sacred cow and not something a core book wants to abolish without very good reason. No matter if you consider it balanced or not, its something you need to consider in the game design. And if there will be more outrage over it or not.
These two principles have so many deep design implications.
For the upper-tier Fighter: It is good to keep combat simple. But ADD complex options for the social and exploratory pillars.
For the upper-tier Wizard: Simplify spell combat. Separate out the combat spells, and simplify the slot system for them. Turn noncombat spells into rituals that dont use slots.
These two class adjustments can help more players extend their campaigns into higher tiers.
While there are solid arguments for and against Fighters getting stuff to do in exploration pillar, and how (feat monkey versus the rogue's skill monkey), that has little to do with extending into the higher tiers; its something that needs to be considered from level 1.
Ultimately, the fundamental problem is that Fighters and Rogues are limited by being non-magical classes and there is a more than subtle bias that says that purely physical classes should be limited by purely mundane ability. Whereas magic classes lack that caveat. A level 16 fighter needs to walk to get around. A level 16 wizard just teleports. A level 16 artificer builds a flying mount. These are not equal by nature of their very classes.
Likewise, not using spell slots is terrible balancing for spellcasters. A huge chunk of being a wizard is spell management. Knowing when to use that sole level 6 slot on True Sight or save it for Disintegrate or Mass Suggestion. Turning everything utility into a ritual means True Sight is always on AND you still have a pocket Disintegrate. This is just going to ensure caster dominance at higher levels and make martial characters feel even more useless.
At the upper tiers, the spell slots of the Wizard and other full casters become increasingly cumbersome and complex. A single pool of spell points cuts thru all of this. Upper tier Wizards suddenly become simple and easy to play.
....
The upper tier spellcasters suddenly become easy to play.
Complexity of spellcasters has never been the issue here. Or, rather, this kind of complexity has never been an issue.
The fundamental problem with high level casters is the breadth of possible abilities makes it difficult to plan a game around. Let me give an example. I remember that an adventurer writer for D&D made a level 12 adventure about a temple full of fiends. The party's job was to clear the place out. If the group had a cleric? Forbiddence. Level 6 spell. Could cover the entire building, prevent teleports, and rather quickly killed all fiends in it. Quest over.
That's the problem - not only can high level casters do things like this whereas high level martials can't, but DMs need to plan entire adventurers around these abilities that will trivialize the story. And that's not always easy. And that's just one spell - there's multiple game changing spells between the cleric, druid and wizard lists that need consideration.