The current campaign is very close to the natural ceiling of fifth edition, that's all. Yes, there are things he can (and is) doing to challenge his players right now, but the bag of tricks is a lot emptier than when it started.
But still has more than enough in it to run adventures for a looooooong time. Uk'otoa, Icathon, the Luxon, The Chained Oblivion.... there are a lot of high level threat cards still in the deck, and it is not being defensive to note that Mercer has shown he is capable of running an excellent game at that level.
Who's lamenting anything?
High level D&D is a different animal than low level D&D. It is, frankly, a lot more work for the DM (and for the players, for that matter) than low level D&D is. It doesn't much resemble traditional fantasy adventures but is closer to magical superheroes. There's a lot less to do at high levels, especially in fifth edition, which doesn't do much to fold in new challenges like domain management (which plenty of people, including me, found boring in previous editions), meaning that, eventually, you end up just a little weaker than the average demigod and given the choice of fighting the same sorts of challenges your last high level game included (different gods, different fiendish leadership, etc., but similar kinds of problems), probably bouncing around the planes, etc.
You ask who is lamenting, then you lament for a paragraph.
Seriously - there is a lot more to high level play than you are offering up here. It is a bit counterproductive to say there are too few options, then to start listing them and end with an etc.
There are a huge number of storylines and adventure options
baked into the basic setting of D&D, as well as into the commercial campaign settings. They're there to exploit. And so much more. Vecna and the Somnovem are not exactly retreading a lot of ground. Neither of those games played like fighting through a gauntlet to the core of Hell, being in the center of a massive war between two huge nations, surviving in an alternate reality, adventuring into the Far Realm, or any of the other pinnacle adventures from any of my campaigns in the past decade.
In contrast, there's a much larger variety of games you can run at low and mid-levels.
Agreed. However, there are far more options available at high level than any DM could run through in decades of games with the same playing group. It takes about 2 years to run through a campaign from 1 to 20 in 5E if you follow guidelines and play weekly (YMMV). If you can't come up with imaginative and fun high level options with 2 years prep time.... try harder.
If you wanted to run a campaign resembling Game of Thrones, high-level player characters are going to fast-forward right through it (and more than King's Landing will be going up in flames, so I hope you don't want your setting to remain intact for future use). Likewise, Lord of the Rings will be a handful of short adventures at most. Even the War of the Lance will be relatively brief, although it'd last longer than the other two examples.
Your comment on intact settings is quite frankly, ridiculous to spout when we're in a thread about Critical Role where we saw a high level game come to a conclusion and then the easy way that it was continued in another campaign.
Further, as a DM that has run dozens of campaigns to high levels
in the same campaign world, I can state that this is absolutely not a problem in the slightest. Some of those campaigns changed the world. Others did not. When the world changed, that was fun for the 'next generation' to explore.
As for wanting to run a Game of Thrones, Lord of the Rings, or War of the Lance at high levels - I've done political games to 20, and I certainly stole from the War of the Lance a few times in my campaigns in the past. They're fine. Lord of the Rings - well, if you want to duplicate it, no. If you want a campaign where a powerful Malignant Force controls massive armies and is trying to conquer the world, I've done that at high level as well. If you're specifically talking about a burglar throwing a ring in a Volcano ... well, no, that goes back to the discussion of not lamenting that you can't run your low level game in a high level setting.
And for people who like that style of play, rock on.
But it's not a personal failing or a failure of DM skills for someone to decide that's not for them...
When what you like or do not like is a decision, is it a failure to explore your options. REMEMBER THE GREEN EGGS AND HAM.
I've run a lot of D&D. I've had a lot of players that join my campaigns talking about how they like low and middle level play. I take that as a challenge, as a DM, to make sure they are engrossed in the campaign to the point where they don't want to stop the story, and they are enjoying their PCs so much that they would not dream of wanting to move on. While my record is not perfect, it is good enough for me to be proud. And, while my approach is not the only good way to run a higher level game, I am constantly amazed that it isn't more prevalent amongst DMs on these boards, Reddit, etc...
Letting the PCs be powerful, allowing them to use the high level abilities, allowing them to do so effectively, and allowing them to pass lower level challenges 'Like a Boss' seems like the most obvious thing in the world to me, but I see soooooooooooo (big breath) ooooooooooooooo many threads by DMs lamenting how unfun high level is because they can't X and Y like they do at lower levels, and how high level D&D can't be fun (despite countless examples that prove them wrong). And then I see so many threads by players that lament their DM ending the campaign because they 'run out of juice' just when their wizard/cleric/etc... was getting a chance to be really powerful...
There is nothing inherently unfun about high level. There are a lot of different ways to do it. If you don't enjoy it, seek out some advice on how to do it a different way. Or don't and stick to low level stuff if you like.... but if you do, understand that you're missing out on the fun side of higher level play ... and stop giving in to the fake news that high level play isn't fun - especially in a CR thread.
