Thanks for the great explanation. I could also go down the B6 rabbit hole and ask what UA is doing to respond to their starting SFO-CUN/RDU/AUS/MCO etc but that can be for another thread.
I honestly don't think UA is paying them much (or any) attention from SFO. Right now, B6 is making moves especially in EWR and that's what's most affecting UA's operations, so that's where we've seen the most response from UA. From SFO, as of this moment, B6's adds really do not seem to matter in UA's eyes. I'll try to break each route down individually - but keep in mind that UA's summer schedules are not at all to be trusted. So I won't compare any frequencies between B6/UA. B6, on the other hand, seems to already have a very modest schedule in place for this summer... their schedule seems more reliable atm even though it's not finalized.
SFO-CUN has been doing decently for B6 especially during spring break, but it's not even a daily operation. It's twice weekly and ends mid-April before returning as daily on May 6th. It was previously scheduled for twice daily starting June, but it looks like it's been pulled down to daily until early September. We'll see if that stays. Not much to see here, as US-Mexico travel will be booming and 1x daily won't be putting any sort of squeeze on UA.
SFO-RDU has been pushed back and pushed back again and really weakened. It's now cut down to a bare-bones twice weekly the entire summer... it's basically inconsequential.
SFO-AUS is now at 4x weekly the entire summer, very low frequency and won't make a dent.
SFO-MCO is scheduled for 3x weekly starting June until September. Traditionally, UA has flown multiple dailies to MCO and I'm sure this will stay somewhat constant, so again, nothing to see here.
SFO-BDL is interesting, there's definitely a market there as UA themselves tried it (daily, seasonal I think?) but cut it a couple of years ago. Really, the only thing B6 is doing is taking away pax that previously would've been flying thru BOS, and this is probably why UA cut SFO-BDL in favor of just flying through BOS to streamline ops.
SFO-LAX is planned for thrice daily this summer... say what now?! IIRC it was among the lowest LF's systemwide when they started the route late last year at twice daily. No wonder they suspended it till this summer, but they have still got the wrong idea. They will definitely be crushed in this market by UA, AS, Skywest, WN, whoever, you name it. 3x daily isn't gonna cut it and people who were previously loyal to them at LGB will not be driving to LAX just to fly them up the coast, hence, they have 0 loyalty on this route, just like many other of the aforementioned ones, where it's applicable.
SFO-EWR is interesting but I'm not sure what to make of it, given that UA is so dominant on SFO transcons. B6 has carved out a market for itself especially with mint at JFK though, and they've already found success at EWR-LAX, so UA might need to take this seriously. Haven't seen them do anything in response to this though, and given B6's poor results on EWR-SFO in contrast to their success on EWR-LAX, I'm still not convinced they actually need to do anything as of yet.
Anyway, the "bottom line" on most of these adds with these current frequencies is that B6 will not have any relevancy in these markets by opening these routes with schedules at a fraction of the frequency that other carriers have historically operated. (except for BDL, which was unserved). Newark is the only other interesting one...