That's a very white room analysis. Non-combat encounters exist and resources can be spent there too. 23 rounds sounds like the high end when level 20 PCs have 22 spell slots.
Even without presupposing things like every caster being a Wizard, Land Druid, or Sorcerer-that-only-uses-SP-for-slots, that would mean a spell nearly every round for the whole day. Even at 8th level, it's still an actual slotted spell every other round at that point.
Further, remember that both per what the designers tell us and what we can just see with our eyes, the game is specifically built on the idea that you need enough rounds for non-daily-usage characters to keep up with daily-usage characters. For the Champion Fighter to even just
keep up, they need their bonus-crit damage to add up to enough to stay in parity with their fellow Fighters, to say nothing of a Wizard or Sorcerer.
Assume 23 rounds of combat, two short rests. At level 7, the Battle Master gets 6d8 such dice per short rest, so, they're getting ~18d8 bonus damage over the course of the day, so about 63 collectively. Presume the Champion is using a Greatsword for 2d6 damage, aka 7 damage. Meaning, we need to get about 9 (63/7=9)
bonus crits, not just the expected ones. 23 rounds + 3 uses of Action Surge gives us 26 Attack actions, each of which generates two attacks, for a total of 52 attacks. Of those, 5% (2.6) are expected to be crits normally, and another 2.6 are expected to be crits from the Champion passive bonus. So...we're getting 2.6 bonus crits, when we need
9 just to keep up with a Battle Master. Even if we assume a third of all Fighter maneuvers aren't used to add damage, an extremely generous assumption, then it's
still only half the amount of bonus you'd need in order to keep up.
Sooo....yeah. You say this is "the high end" when actually it's
extremely low. That's a huge issue right there. People just do not understand how much combat you need in order to help non-casters keep up with casters, or short-rest-based classes keep up with long-rest-based ones.
Yes it's true challenge rating was calibrated with a bad baseline (medium difficulty). The question is whether monsters can be redesigned to salvage 5E. I've seen Matt Colville and others design them with more abilities to balance the action economy more, like having bonus actions, reactions, bloodied triggers. Legendary actions are basically mandatory for solo monsters.
It feels cheesy to just pump saving throws and HP and tack on condition immunities etc. Changes are probably needed at fundamental levels so interactions feel more dynamic and fair. Most monsters don't have nearly as many resource types as a PC near their level, which makes no sense if it's supposed to be a threat to multiple PCs.
Of course it feels cheesy, it's the equivalent of "balance everything by making it all the same". But yes, fully agreed, fundamental changes are nearly guaranteed to be required to address this. Patches can help individual issues for individual tables, but as so often happens, they prioritized the short-term bump at the cost of long-term stability.