Having worked on federated login with Sony, I suspect this "account taint" behavior is because Epic is using Sony's payment platform for loot crates. It's against Sony's policy to be able to pay for things to be used outside their ecosystem...mostly because that opens up a whole can of legal worms. It's for the same reason Spotify subscriptions use in-app purchase but you put your credit card into Uber and Lyft. Allowing cross-play means the in-game items you paid for with loot crates should transfer.
Aha, HN devs say, why don't you just carry a bit with those items that were paid for and hide them on other platforms? This is, more or less, how Blizzard deals with this. However, if that's not how you set it up out of the gate, that can be a painful thing to do retroactively. That's why I feel it's a bit disingenuous to heap ALL the hate on Sony. Epic could possibly work around this.
That said, everything about the account is cosmetic-related. You'll have to start all over getting your John Wick skin...but how big of a deal is that really? It doesn't affect gameplay. That's probably what Epic is thinking.