I am not sure about how the market shrinks or expands. There may be a lot of reasons people may need a computer instead of a phone, like even writing emails is not that convenient on a phone to do it on regular basis. People may still do it on a tablet with an external keyboard, and theoretically they could also use an external keyboard and screen on a phone, but phones are not really designed for that for a non-techy person (I have tried to use an iphone in this way, it was a horrible user experience due to iOS). Smartphones are chastised devices. They could have a lot of potential if they were actually letting people do stuff on them, but that's not what they are designed for. They are designed for serving content passively while giving as little freedom to the user as possible.

Mini PCs have the same problem like laptops, in trying to squeeze performance in a small form factor which then poses a heat dissipation issue (and even more because the adapter is typically inside the form in this case and that results in more heat). And you cannot put the same high end gaming gpu in a small form factor, which is an extra characteristic they share with laptops vs larger desktops.

Also macs are fine with gaming if the game actually runs. It would not be my primary choice or suggestion if looking specifically for a gaming machine, but I also do not need to look for another machine for gaming now that I have a macbook anyway, as it runs games good enough at high end settings. But a mini pc would not have been a suggestion as gaming machine either.