I really doubt it will reach PS2 numbers unless the PS5 is a repeat of the PS3. Even then, I still don't see it.
But congrats to PlayStation 4 for reaching the elusive hundred million club. It's so close to surpassing the Wii and PS1. It could surpass Game Boy (Game Boy Color sales included, as most of you know), but I don't see it reaching DS or PS2 levels. I don't think that will ever happen again in the industry with gaming PCs and smart devices competing for consumer's wallets and attention.
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: 122 million (was 105 million, then 115 million) Xbox Series X/S: 38 million (was 60 million, then 67 million, then 57 million. then 48 million. then 40 million)
Switch 2: 120 million (was 116 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