Well I doubt the active userbase for PS3 was anywhere close to 10m in 2014/2015 but yes that is something that needs to be taken into account, however I believe my point still stands that multiplatform/cross-gen sales take a while to transition in Japan.
Talking about active userbase only reinforces the point that Switch games should outsell PS4 games ;). Of course multiplatforms will take a long time to transition and will possibly never transition. Even in the US when Wii/360 had a massive userbase advantage over PS3, many japanese games sold better on PS3. Userbase and demographics are just too different for PS4 and Switch, it is unlikely that the demographics will change without a big push from either publishers or manufacturers. For example we saw a positive climb with the 360 and pandering to the japanese audience but then a grand collapse when they did the opposite with the XB1.