You're correct insofar as it wasn't a perfectly analagous situation due to the trade, but you have to acknowledge that many thought he'd turn right back around and sign here once free agency opened. Didn't happen.
I guess I'd ask for an opposite example - when has a top tier guy ever hit the open market and then opted to re-sign here? There must be an example but I just can't think of one. I feel like we will have to debate what "top tier" means before we find someone who fits.
You will not find a lot of examples because the Sox have tended to extend their young players before they hit free agency (Pedroia, Youkilis, and now Xander). Or they were OK with their departures (Ellsbury, Damon, Kimbrel, Papelbon, Jason Bay, and even Pedro). Or they were traded (Nomar, Lester, Andrew Miller, Nick Punto). The Sox probably should have resigned Beltre, and probably could have had they been more aggressive, but it was clear the Sox were OK with him leaving as well. The Lester situation was poorly handled, as there was still some lingering resentment from the absurd 5/70 offer the prior offseason. But the Sox have significantly revamped their front office since then. This is a very different front office from 2014.
Varitek and Lowell resigned here. They weren't what I would call "top tier", but Varitek was highly regarded among catchers, and Lowell had just won World Series MVP.
There is obviously risk in letting a player get to free agency, and that risk is greater if the player was willing to sign an extension earlier. Greener pastures are a thing, and often times free agents across MLB have noted that their prior team had a chance to sign them the season before but decided not to. So far, Betts hasn't even entertained an extension. But that could change this offseason.