Paying for ps+ has little to do with the "quality of online play" most of the time, the services are just a glorifed matchmaking service, with no game hosting going on. Take two of the most popular games, COD and BF. Call of duty is p2p for the most part and while Battlefield has dedicated servers, they are hosted by EA, neither has anything to do with your ps+ fee. Microsoft is fairing better of that front, with more games using Azure, but last I heard studios/pubs still have to pay out of pocket for the servers, so again xbl is just being a glorified mm service, rather than being the reason you have those servers.
Rocketleague, a cross platform game, everyone using the same Psyonix hosted servers, free to play online on pc, xbox/ps, nope, even though it's the exact same gameplay servers.
The real reason you pay is simple. You pay, because MS/Sony know you will. No sense leaving that revenue on the table, when it's there for the taking.
Meanwhile, Xbox live and all it's services are 100% free on pc on games that support it, because the last time MS tried to charge for it, PC gamers rightfully told MS to jog on.