Offer sheets tend to be tough to make sense for the team+player, but I always wondered if more backup goalies would consider that route if they were offered starter money elsewhere. Florida preemptively gave Spencer Knight a 3x4.5 extension since they were worried that some teams would approach him this offseason.
A 4.2 mil AAV offer sheet would have required a 2nd rounder as compensation this season. That number will go up slightly when next season's cap is announced. But Swayman could very well be happy to stay in Boston and decline any offer sheets (but also use it as leverage).
And the other difficult thing with offer sheets is that the original team has a week to match. So if you offer sheeted a goalie on the first day of free agency, you might lock yourself out from the rest of the UFA goalie market. If the original team matched, then there might not be much left worth signing.
San Jose gave Boston a 2016 1st rounder for a 25 year old Martin Jones who only had 34 career NHL starts to that point but had a decent AHL track record. There seem to be indications that the 2024 Draft is looking a little lackluster.
TL;DR - The goalie RFA market might be more closely tied to the draft pick compensation levels than for proven NHL players.