Well, Spencer said already last year in a podcast that it's not about how many first party studios/games you have but about how many exclusive games you have. If you have 20 exclusives and all from first party or 20 exclusives of whom ten are first and ten third party doesn't change the amount of exclusives.
He even explained why it can be better in a case by case scenario and it makes sense that it can be also a problem when you have so many own studios who all need buildings to work in and employees to pay for.
The only real problem for Microsoft is that PS4 has so many more sales that it is obviously more expensive for Microsoft now as if Microsoft would have a 2:1 or even higher lead since Microsoft has to compensate for all the sales the third party developer would expect with a release on PS4 but in general it's not automatically bad to have less own studios or a weaker first party.