I think every successive generation had proven that there can be 3 successful consoles on the market at one time.
You have to believe that Microsoft must want to regain the NA market over Sony in this upcoming generation, they have certainly gotten most of the conceivable mistakes out of the way by now.
I expect the next Xbox launch to be very strong and compelling.
Nintendo has a hit with the switch much like they did with the wii, so all they have to do is not screw up the naming of the successor and they will be golden.
PS5 will probably be in a dead heat with Nintendo for worldwide sales (launches aligned) if both companies play their cards right.
Microsoft will dominate in NA, and languish elsewhere despite some initial efforts to the contrary.