My bet is
PS5 - 80m-90m
XSX/XSS - 40m-50m
by the end of 2025

I can't imagine the market itself is smaller this next gen than it is now. A Switch Heavy might throw a bit of a wrinkle into the numbers, but I also can't imagine that will be powerful enough for the games. I won't predict numbers but I think the split will be closer to 60/40 than the 70/30 it is currently.

Edit:

For those wondering why I don't think the market will shrink (unless a sustained recession hits) is that I think the tablet/phone damage has mostly been done.  Population and wealth growth should sustain and/or increase markets from here on out.  The damage from streaming gaming will come in the future.  We aren't quite there yet.  What damage you may ask?

The market size in the Wii+X360+PS3+DS+PSP was 101.6+85.8+87.4+154.0+80.8 = ~509.6m units.  Although not the final picture the current snapshot of the current market is WiiU+Switch+XBONE+PS4+3DS+PSVita is 13.6+64.8+48.3+113.4+75.8+15.9 = ~331.8m units.  Even if 100m more units sell (mostly to the Switch) that is still about ~1/5 reduction in market size.

If you drop the Switch for the next gen (if you're one of _those_ people), then I would expect the Switch+XBOXS(S/X)+PS5 market size still be roughly ~350-400m units. 

I don't know about how much each manufacturers will sell but Sony will have competition this gen🤗
https://youtu.be/TNpwz3E-_-4

PS5 90 million
Xbox series 60 million

35 Million Xbox Series
75 Million PS5

If its a five year generation, I'm going with 95 million for PS5 and 60 million for XBS/X.

After Zenimax acquisition I need to revise my predictions a bit:

PS5 - 105m
XSX|S - 80m

Imagine all of the doom articles we would get if these numbers turned out true 

Best case scenario we see a 360/PS3 ratio, but not identical. I’d say Xbox 85, PS5 100 million.

Keep the exclusives coming, keep Bethesda games off PlayStation, stay competitive in price and services and they’ll take back some of their lost marketshare. Nm PC and Android, their strides in those spaces are all a bonus that go beyond the console war. 

Anyone predicting Xbox to sell less then Xbox One I won’t begin to try to understand. They got the power, price, value, and dun dun dun....the 1st party narrative on their side (23 to 14 studios). Xbox One had none of those narratives at launch. 

It's hard to say. At this point in gaming history the Brand Power is more evident than ever. If Xbox stays true to its plans for Series X/S, they will definitely do better in non US market, but with Sony and Nintendo seemingly still staying strong, they'll still remain 3rd in line.