Ok... so you predict the XSX/S to sell about the same amount as the XB1... and predict that the PS5 will sell 20M - 30M consoles less than the PS4.
Basically, you are predicting that coming off the PS console with the most momentum a PS console has had since the PS1 - PS2 and the PS2 - PS3, the PS5 (barring any $599 surprises) will do only as well as the PS3?
For the Xbox Series S/X I think the S and it's price point will really help get a lot of sales. I think if the price point and marketing is on point it can bring Microsoft back to winning NA and grab some of the EU market. I even think they'll do better in Japan this time (which isn't saying much). With that I believe it will sell around Xbox One numbers.
For PS5 the reason I think it will sell less is that at the start of the generation Nintendo and Microsoft stumbled and Sony just capitalized, which lead them to essentially becoming the "default" console. Since Microsoft and Nintendo are in much better position I think it will hurt Sony. On top of that PC gaming has become way more popular now than ever before which will lead to less console sales. Japan is done with game consoles, so PS5 will not sell as well as PS4 there (I predict 6-7 mil). Game streaming I think will become more mainstream in a few years, which won't help. All of that is why I think PS5 will be around PS3 numbers.
Of course I could be wrong and the PS5 will do 100 mil and the Xbox Series S/X will do less than 40 mil. This is just all predictions here just like with the Switch where some thought it would fail. I predicted it would do over 100 mil and it seems I'll be right on this one.