Things don't have to happen in the exact same sequence for the result to be similar.
Putting your first party games on other platforms devalues your own. That means that there's going to be a significant segment that doesn't engage with your platform as much if at all given the choice, and it means there's a narrative that says "there's no reason to buy x". The second someone, anyone can make that legitimate claim, you've lost perceived value. Even people who have no intention of moving over the PC will just see the first party platform as less valuable and less exciting.
And then there's the production side disruption. There's already speculation on this forum that Sony's games have been or will be compromised by the "necessity" to make PC ports easier down the line. If they go day and date, the ceiling for technical polish that these games used to enjoy will get lower.
And for what? It's not like anyone who has a strong enough PC to justifiably want to play games there as opposed to PlayStation didn't have "access" or something. You're not increasing the audience size, you're spreading it out. Do people not realize that third party games on everything down to a toaster rarely sell better than the heavy hitters exclusive on PS4 did?