I think the Switch (officially Gen 8) will sell somewhere in the 90-100M range. Its fortunes will not be impacted by the arrival of the PS5 & Xbox 4.
The PS5 and Xbox 4 will, assuming neither Sony nor MS bungle anything major, do about even in the U.S., while the gap in Europe will be a bit closer, and Japan will remain about the same. I think this would translate to around 100M for the PS5 and 70M for the Xbox 4, barring any emerging markets starting to adopt consoles en masse. I guess we'll have to wait until the consoles are officially and fully unveiled to make any other guesses.