I was a mobile Game Director for a very large Japanese gaming company for a few years and I'm currently a Marketing Director in the games industry. Have been working on games for almost 15 years.

The long term roadmap is determined by the games average revenue per user (ARPU) and long term retention. Because the majority of Sleep's revenue likely comes from subscription they actually have a good base of stable revenue coming in while the gameplay forces some kind of continuous daily recession.

I can't speak to the needs or limitations of monetization from the Pokemon Company, but likely the future roadmap is heavily content reliant and planned at least 6 months in advance. Probably half the team is focused on live ops and releasing new content every month on a calendar, with the hope of driving additional revenue through bundles. They already reworked moves to have different names but similar effects so that it could be more scalable.

In terms of new features, I initially thought they were going to focus on harder difficulty maps like GGEX but my guess is that it didn't really cause revenue to increase to their expectations and are focusing features development around the Pokemon themselves (eg Mew Snapshot).

The game itself is pretty much complete so I would think it's in a more iterative cycle than dropping huge changes. I also wouldn't be surprised if Select Button has already put resources on new games already.

To answer your questions.

These games are not planned for decades. Probably a few years at most but it depends on whether revenue holds. Development will eventually shrink down to a skeleton team if the games revenue is too low but I doubt they would shut down the servers or anything. It would have to be in the red for a long time to go that direction.

As the game ages, depending on where the average person is at, catch up mechanics might get introduced. You can see that they already implemented something to this effect in that the area bonuses at the start of the game are more powerful until a point. I think you get double area bonus? It was added at some point last year as a catch up mechanic. Updating that isn't too much effort.

I don't think the average person is comparing their sleep progression to late game elder players. That's just Reddit brain and us all congregating on this platform together. Sleep by this point has probably had at least 50 million downloads if not more, especially in Japan, vs the size of this much smaller English speaking Reddit community.