I think honestly they might be looking to have their own platform on PC one day to not paying the platform fee.
if they keep using steam backend for part of their PC framework, they will pay a lot more in the end, compare to invest now take some flames and properly push for their account backend on PC WHILE have steam's market place/distribution work for them.
Don't get me wrong, Sony already have the capability to server digital contents to all the PS5s, but it's a closed system and no worry about hacking etc as much compare to PC.
It's the end point modem+router. It's like saying removing the landline phone from people's house without destroy the exchange boxes would cripple communication. It's not cause they are easily replaced.