There is no "need" with video games. They're a luxury. If games are available on multiple platforms, you make your choice based on a variety of factors. Are you telling me you cannot think of any reason as to why someone would choose to buy a Playstation except for 1st party games?If all the playstation are available on pc, why do you need to buy playstation?
If you pc that can ran with better graphics, why buy playstaiton?
If online on pc is free? Why pay for ps plus?
If you can buy game on Steam summer sales for $5, why buy games on playstation for $70?
If theres a great exclusive games on pc that you cant get on playstation, why buy a playstation?
"If your PC is better, why buy a Playstation?" Well my PC is better than my Playstation, but I have a Playstation so I can sit on the couch. So I can play it on a 55" TV. Because, compared to other Platforms, I like the UI and the controller, because I have friends on that Platform. Consumers may make this choice based on value for money, or having a fixed amount they can spend.
You pay for PS+ if there is a network effect and you have friends that you play with. I don't generally pay for it because I don't play online games usually, but same thing with Switch online and MH Rise, just pay when you want to play. If you're poorer and can't afford multiple games, cost of a sub to get new games every month also a reasonable decision to make.
Playstation also has sales. Though Steam has a much better refund policy, so it's definitely better to buy something on Steam than PS and is a fair point of comparison between choosing platforms.
Again with your final point, you ignore everything that a PS is beyond what games it plays. The majority of the market is 3rd party games, for consumers the choice is holistic of everything I've mentioned. I bought an Xbox one to try Sunset Overdrive out on release and the Xbox one SUCKED in so many different ways: batteries for controller, controller had weird edges along where palm gripped, UI was terrible, and so on. These are all differences that persist regardless of game availability.