Why would that be the case? If like you said, and say one of their GaaS projects ends up succeeding. At that point, Sony would have to throw considerable resources at making MTX related material. Aside from that, Sony would look at what made that specific title successful vs their other attempts, and pour money into repeating that process.I would argue that the exact opposite would happen. Sony are investing this much in gaas titles because there is no guarantee any of the projects stick the landing...but if only one of them...or maybe a couple succeed in becoming money printing machines, I think that that would make them concentrate on updating and nurturing said titles, while using part of the proceeds to pay for the expensive SP products; New gaas IPs would be introduced sparingly from that point onward. If Sony succeeds in their gaas endeavours, we will be seeing more big budgeted SP franchises, not less.
If they make a successful multiplayer GaaS title that has the ability to print money. They're going to try and repeat that process in order to print even more. Them taking those proceeds and throwing them at a singleplayer game with far lower potential wouldn't make sense.
Just like Epic abandoned Fortnites singleplayer mode, and just like we haven't seen a new GTA release in over a generation... If they strike gold, Sony would pivot towards creating more multiplayer at the singleplayer's expense.