I Say this because Microsoft dont get games that Sony gets like the many anime fighting games and street fighter 5 and the enourmous amount of Exclusive PSN games . I t doesn't matter if Microsoft has 16 first parties Sony has more japanese support than Microsoft while American devs do both . And the many PSvita japanese devs going to PS4/PS5 while those PSvita devs and others ignore microsoft . Unless Phil Spencer convinces those Japanese devs so far :(
Having another studious is not bad either, you also need to remember, Sony attention now are divided into two platform, which are VR and Mobile devices (smartphones ). Also Sony capability on making first party ip in a year is limited to just one or two ip. With having another studios they will likely to produce more in a year. Of course they can just ask 3rd party studios to make exclusives games for them but having a studio that is inside your policy will be much better. And also like u said Japanese developer can be bought by money especially Microsoft , so . Having more is not a sin.