As someone with both a passion for and a vested interest in solving all the issues of Epic Tier (and above) play - to the extent I am writing some books on the subject (GOD RULES Player's Guide coming soon), I fully believe all the issues are easily fixable with the exception of "We need MOAR!"...as in we need more monsters, more magic items, more boons, more adventures, more epic locations etc. As a lone creator (albeit working with multiple artists) writing a big book takes a long time. You might well respond "Well then write a short book" but some topics just require x amount of pages to do justice to them and Epic/Immortal Tier play is, in my opinion, one of those topics.
- The leveling system takes too much time IRL to reach high levels
Leveling up is relatively rapid in 5E. However, one element I have borrowed from 1st Edition is that slaying a Demon Lord or Deity permanently nets 10 times the amount of XP. So Slaying Orcus permanently would be worth 900,000 XP etc.
- The number of things a PC can do gets overwhelming
True, but that should be offset by the players themselves being experienced enough with the game. Unless you are simply starting at 20th level with new players - which I don't recommend.
- DMs aren't interested in using high CR antagonists like demon lords
I disagree. I think, as many others have pointed out, there simply is not enough diversity of Epic Tier monsters to make Epic Tier campaigns interesting. I mean at least the new 2025 Monster Manual made a slight effort in this regard, but I think to flesh out Epic Campaigns (as opposed to simply an adventure or two) you need more than a few big monsters (welcome as that is); you need a mix of new epic races with support structures, epic templates and some monster building rules (even if those are narrowed by specific sub-types).
- High level PC spells make the game harder for DMs to account for
This is true, but I think with a mere handful of fixes the worst offenders can be mitigated.
- Players lose interest in PCs and want to make new ones
Given the proliferation of wishes, the ability to 'RESPEC' should be an option on the table.
- DMs lose interest in long-running campaigns and want to make new ones
This is always going to be a factor with any game. But (and someone else mentioned this earlier in the thread) always try to up the ante.
If any of you read manga, one of the ways these stories keep you hooked for years is by consistently upping the stakes and introducing new threats. Of course the immediate problem is that in D&D you cannot really do that because you run out of newer more powerful threats very quickly.
- Other (please explain in post)
The other major issue is
Game Balance. But ironically I believe that is easier to fix once characters progress into Immortality, because when PCs have the safety net of 'death' being a mere speed-bump, it allows the DM to really go to town with the difficulty without worrying about potential TPKs. But I will discuss the specifics of this in my book.
Overall I think just having rules, monsters and adventures out there will both generate interest in and just as importantly facilitate Epic Campaigns for the groups that wish to try them.
20 years ago I released the Immortals Handbook: Epic Bestiary for 3rd Edition. In that book I included the Neutronium Golem which had a Challenge Rating of 9721. It was a bit ridiculous and far beyond all the other epic monsters in that book....but people loved it! Even though most people probably never intended to use it (just like most D&D campaigns don't use the Tarrasque) just the simple fact that such a monster existed got them curious, smiling (at the craziness) and invested.
...or to put it another way "If you build it, they will come."