Couple things for context here.

1) You will be halfway through your career in 2045 ... when the youngest baby boomer is 81
2) The 747 had a 54 year production run. If the 787 matches that, it will exit production before you retire.
3) You have 42 years ahead of you. 42 years ago, 707's were still in service with the majors.

All the "delta this, united that" stuff is speculative for the 2020's. For the 2030's and 2040's and 2050's and 2060's where you will spend the bulk of your career - NOBODY HAS A CLUE

So do whatever ... but don't think you have any idea what's the right choice.

I'm in the just move close to JFK camp, but if you are convinced you want to die in eastern PA, it's not at all unreasonable to switch after a few months at Delta.