One thing I’ve picked up from my Dad’s long years of being called by only his surname derived nickname is that eventually people forget you have any other name. Especially if that’s how you’re introduced - and it’s usually the given name they lose first.
So imagine someone coming to the Haus or hockey practice or just some place where the team is gathered, long years in the future (or even Jack calling Bitty Eric because that’s his name and they are boyfriends after all) and asking after Eric and everyone going ‘Who?’. And Bitty puts his hand up and is like ‘That’s me?’
‘Your name is Eric?? Since when???’
‘Since forever?’
“Yo brah, I think Jack is secretly dating dating some dude named Eric!”
“Hey Bits, do you know anyone named Eric?”