I know that there were ten Pokémon Mystery Dungeon video games released in Japan, including seven them also released internationally, and eight of them are developed by Chunsoft, and the last two of them were developed by Spike Chunsoft. However, I'm just wanting to know why you think Spike Chunsoft had no longer been involved developing anymore Pokémon Mystery Dungeon video games during Generation VII and later? Please let me know, thanks.
If you want to take sales figures at face value, then the answer probably is that PMD, like most spinoffs, didn't really sell nearly as well to keep justifying its development. If you want to give the spinoff series a benefit of a doubt and even claim that the sales figures listed are outdated, they likely aren't outdated by a whole lot. Also it's worth keeping in mind that a lot of these spinoffs were released during a time where mobile gaming didn't storm the world as it seems to do these days, and yet they still didn't do that well, which is probably something to keep in mind.
As much as I personally want a future PMD though, I'm just unsure of the demand of it. :\ It seems that successful Pokemon games (looking at GO, here) integrate Pokemon GO to some fashion or another to ease newcomers into the franchise, and PMD is an entirely different thing altogether that has a different learning curve (although how much of it depends on the game) as well as different mechanics to learn. If any future iteration is going to be released on say, the Switch, it's probably going to be an eshop exclusive for like $30 and no more than that.