Justin Alexander at the Alexandrian blog has had a similar line of thought to myself in reaction to the news that WotC is planning to drop some domain-level play rules for 5E (or One D&D, or whatever).
And it seems from the comments of Justin's blog that a lot of people don't really conceive of how to run that stage of the game. Justin himself seems to get it, but the focus of his post isn't on how to manage that tier of play, so he doesn't really get into it.
I haven't run a domain level game for a long time, but I still remember the things my friends and I did that made it work for us. Even as kids, with little real experience or expertise about politics or war or project management, we were able to understand the rules and make them work for us.
So first of all, my advice to anyone wanting to run a domain level game, or transition their normal game to domain level play when the PCs get more powerful, would be to get the Mentzer Companion Set, or a game like Adventurer Conquerer King that goes into actual rules and systems for this sort of thing. I'm not that familiar with ACKS, but I am with Mentzer's Companion rules (with some extra useful bits in the Masters Set, also most of it is compiled in the Rules Cyclopedia).
Or, pick up Chanbara. I've got a version of domain management rules in there!
I'm sure there are other games with rules for setting up domains and managing them. Find one you like. It doesn't really matter which one. Or make up your own. It's just useful to have some solid rules for determining the size of a barony, its population, resources, etc. And then rules for how well you're managing it over time. This is one of the areas where I find BECMI superior to AD&D, as AD&D seems to default to just giving you some passive income and the DM figures out the rest on their own.
Next, several commenters over at the Alexandrian seemed mystified about how to have the party remain a party at high level.
Here's the trick. They mostly don't.
Sometimes, there's a quest or a threat that requires them to band together. That's a typical session or adventure, only scaled to high level PCs. But they're rare at this level. That doesn't mean you have to slow down the schedule of actual sessions of play. It just means that more time passes between quests or adventures. You are keeping STRICT TIME RECORDS aren't you? This is exactly the sort of thing those time records are for. How long will it take the Lord to train up the new recruits? How long will the Wizard be out of the action crafting a new staff of power? What resources does the thieves' guild have on hand to assist the Master Thief?
Most of the time, the DM will be meeting with players separately to manage domains, or else devoting a segment of each session to domain upkeep and management with each player before getting to the adventures. Sometimes, an entire game session may just be this domain management. That's OK. Handle the management this session, then have some epic quest or killer dungeon to tackle the next.
In fact, these days, with easy shared internet resources like a wiki or Discord server, it should be pretty easy to manage each player's domain online between meetup sessions, saving the weekly get-together for more action packed adventuring.
Also, again, read the Companion Set rules carefully (hopefully other rules systems will suggest the same). The PCs don't all have to set up independent strongholds. High level Fighters can become lords with their own castles, but one could be the "landholder" while the others are "wandering" and become the Knights in service to the Baron, and the Paladins and Avengers could serve the Baron and/or the party's Patriarch (if alignments match). Each of these wandering Fighters may have an estate (small castle/tower/outpost) that supports the main landholder Fighter's castle. The Patriarch's stronghold could be part of the Baron's castle, or another satellite stronghold within the barony. Maybe the walled town within the barony is ruled over by one of these other PCs already mentioned, or by the Master Thief of the party. Or the Thieves' Guild may be based in the main barony castle's dungeons. And the Wizard may set up a tower within the barony, or may be the baron's Magist with a tower in the main castle to do their research from.
This way, the players are all invested in the fate of that one barony, and are working together to grow it into a County or Duchy (with the other PCs as barons, earls, knights, etc. under the "leader" PC), maybe eventually a kingdom or even empire.
And of course, if the players each want to have their own independent fiefdoms, that's fine, too. It will just involve more solo or small group play to manage each one of these areas, as mentioned above. And when a threat arises in one of the domains, like a dragon awakening from centuries of slumber or a necromancer's undead army invasion, it's only natural that the ruler of that particular domain calls in their old friends to help manage it.
Finally, as DM, you need to shift your focus for running the game. There needs to be less of an emphasis on dungeon crawling for the sake of dungeon crawling -- although that will still happen, because in my experience, the income from taxes won't cover expenses, and PCs will want to supplement their domain's treasury from their adventuring earnings.
There needs to be a focus on the politics of the land. Dealing with the despot king, or land-grabs by hostile neighboring rulers. Monsters or humanoid hordes invading. Peasant revolts. Diplomacy. Finding NPC specialists to hire (the Companion Set again has a great list of retainers needed by a fief, including salaries they typically require, in addition to those in the Expert Set). Dealing with natural and unnatural disasters (which the Companion Set suggests how to deal with, although this isn't as fleshed out as it could be).
But it's not all defensive actions. Players will most likely want to use their own armies (again, the mercenary costs aren't just there for a way to drain funds from PCs) to expand their territory. The Companion Set has the War Machine mass combat rules, but AD&D's Battlesystem or Chainmail can also be used, or some other set of mass combat rules.
My friends and I had a great time with all of this stuff. It didn't supplant more typical adventures. We still had plenty of those. But as I mentioned above, there was more down time between adventures, and we found ways to mix in the domain management with the adventures for that tier of play. And we also would often roll up new, low level characters to get some dungeoneering or hex-ploration gaming in.
Also, remember, the Domain Game can be the end game, but it doesn't have to be. Again, the Companion Set suggests ways for PCs to continue to be murderhobos wandering adventurers at high level. It's just that instead of focusing on Dungeons of the Week or hex crawl exploration, you're more likely to be enticing the players into epic quests for artifacts or planar exploration, or whatever. But that's a topic for another post.
If you want to play the Domain Game, it can be really fun and rewarding, but both the players and the DM need to shift their conception of what the game is for it to work.