As others have noted later in the thread, while handling power scaling is A reason that a campaign might falter, its more the minority. The simple truth is, high level campaigns usually take a lot of IRL time. Campaigns die by attrition, or because the campaign is resolved successfully earlier, or because the group wants to do something else....those factors are a lot more prevalent than "high levels is difficult to run".
So while you could certainly do some work to clean up high levels, its not going to result in a significant increase in high level campaigns.
Unfortunately, I can't game design more hours into the day or days into the week in the real world. The only solutions I can offer have to be within the confines of the game itself.
If a game falls apart because of DM-Player mismatch, no amount of game design will fix that, either. Nor is there a way to design the game to be played during wedding receptions or tragedies or on moving day so that you don't have to miss a session.
Trust me, if I had the power to design a version of D&D that could somehow be played regardless of external issues like problem players or scheduling, I would leap on that so hard Gygax's head would spin in his grave.
But it's just not possible. So I'll offer what suggestions I can, instead.
one fine solution is to remove all 6+ level spells from the game.
you can keep uscasting if you want or not,
maybe some iconic 6th level spells can be moved to 5th level,
Arcane gate, Create undead, Mass suggestion, Harm, Heal, Summon fiend True seeing are all good 6th level candidates to be moved down to 5th level with some reduction in power, duration, range or other things.
if there is no upacsting, the higher level slots can be turned into lower level ones:
View attachment 412525
or just use Spell-Points(SP) to save some hassle.
Personally... I'd just keep 6th level spells as the 'high end', myself, if you're going that direction. It's the last level where the scaling still makes sense with spells like
Chain Lightning or
Sunbeam.
Though... if I had -my- druthers...
1) 1st level to 9th level spells would be granted by level 12.
2) The list would be shifted -sharply- to cut out stuff like
Simulacrum gaming. A bunch of spells would simply -not- exist, anymore. Or be so drastically rewritten they need to be renamed.
3) The progression of spellcasting would be way more linear in it's actual function with breakpoints clearly defined and balanced against each other by scaling damage values that maintain power parity even through upcasting (a 7th level
Fireball should do as much damage as a 7th level
Delayed Blast Fireball, it's just the other benefits that make
DFB useful over regular Fireball)
4) Class-Separation results in different 'max level spells' for different classes with Clerics/Druids casting up to 7th level spells, but 7th level Cleric/Druid spells being as strong as 9th level Wizard Spells so there's a different progression of power between the two where the Wizard is smoother but has less room for class-benefits while the Cleric gets more class-benefits but larger scaling jumps between spell levels.
It would a disaster a of Biblical proportion. Real wrath of god type stuff. Fire and Brimstone coming down from the sky, rivers and seas boiling! 40 years of darkness. Earthquakes! Volcanoes! The dead rising from their graves! Human sacrifices! Dogs & cats living together...mass hysteria!
Maybe someday I'll sit down and write the "Ultimate D&D Variant" to go on the shelf with everyone else's Ultimate D&D Variant to collect dust.