There are too many double standards around here. Why is it ok for Delta to burn money on their pet projects in SEA and BOS duking it out with AS and B6? How many times has Delta started and stopped flights like SEA-KIX? Three times now? But no criticism for that it seems.
SEA and BOS are both very wealthy markets with strong international travel demand. In 2018, the Boston MSA ranked 8th for GDP; Seattle was 11th. Tampa Bay, by comparison, was 24th. The incumbent carriers in SEA and BOS didn't have the ability to fly their own intercontinental long-haul routes. AS isn't a low-fare carrier. And, FWIW, Delta had a strong historical presence in Boston dating back to their purchases of Northeast and the Pan Am Shuttle. Delta's cutbacks at BOS in the aughts (in no small part due to their bankruptcy) helped B6 to grow BOS into a hub; in 1999, DL/Connection/Shuttle/Express served about 30 destinations from BOS.
United brings nothing to the table at TPA apart from higher costs. Sure, they could offer long-haul flights but there's not a lot of demand, particularly for expensive business class seats which drive profitability.
NWA hub at SEA? MSP, MEM, DTW, NRT, PEK does not make a hub.
NWA had a longstanding partnership with AS at SEA which continued after the DL merger and until their messy breakup -- so DL had a good understanding of the traffic dynamics and demand at SEA.
I don't think there are ANY DL domestic connections flowed over BOS and few over SEA
There are a few (or were pre-Covid), but not many -- the BOS flying was overwhelmingly focused on O&D markets and secondarily feeding the Europe service. DL didn't add BOS-ORD for connections. If you really, really want to fly EWR-BUF on DL, BOS is as logical a place to connect as DTW.