Nintendo Switch vs Nintendo DS (33M - JAPAN): It has a good chance of beating the DS in Japan, but it's not a given. 30 million is on lock, since it's already at 26.80 million.
Nintendo Switch vs Nintendo DS (154M): Even more contentious than the DS/Switch Japan battle. Very possible that Switch is able to match or surpass the DS, but not a given.
PlayStation 5 vs PlayStation 4 vs PlayStation 3 (117M & 87M): PS5 will of course pass the PS3. PS4 is more questionable. I think a worst-case scenario for the PS5 is 95-100 million. And realistically, it will be more like 105-112 million.
Xbox Series vs Xbox 360 vs Xbox One (86M & 51M): Xbox Series would have to fall off a cliff to not surpass the Xbox One by at least a few million. It should finish somewhere in the 60-70 million range. I don't think it's touching the Xbox 360 globally.
Xbox Series vs Xbox 360 vs Wii U (1.66M & 3.33M - JAPAN): It's on pace to sell over 1 million. I don't think it will match the 360 unless Xbox changes their strategy significantly in Japan (and they have been changing it somewhat). It's not touching the 3.33 million of the Wii U. Even 2.5-3 million for an Xbox in Japan would be a great feat.
Lifetime Sales Predictions
Switch: 161 million (was 73 million, then 96 million, then 113 million, then 125 million, then 144 million, then 151 million, then 156 million)
PS5: 115 million (was 105 million) Xbox Series S/X: 48 million (was 60 million, then 67 million, then 57 million)
PS4: 120 mil (was 100 then 130 million, then 122 million) Xbox One: 51 mil (was 50 then 55 mil)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima