I think at least part of the reason for the decline in PS+XB number is not due to a decline in player base, just a decline in purchases. PS2 lasted such a long time there will be a significant portion who bought a 2nd console, some due to failures & some just because slim model was cheap & far less bulky.
PS3 & XB360 probably had a smaller player base than PS4 & XBO, but the higher failure rate meant more players bought replacements. (PS4/XBO sales probably could still have beaten 7th gen but their player base has been cut short by the lack of late stage price cuts & supply issues)
So while the number of unit sales has declined, I suspect the number of owners has either remained consistant or maybe increased slightly. (probably somewhere between 150-160mln)