Judging by their lack of interest in support, etc, I think their business model is based on volume large scale manufacturer supply. So they don't flash a boot loader by choice. (Avoids all that messy dealing with amateurs...):
we have a product using >10k Winbonds/year and get zero support when we ask.

If you want support then, in my opinion, NOBODY beat SILabs. No question of mine has gone unanswered for more than a day (OK, sometimes a few "clarification exchanges" have been required). Another "SILabs feature" is that, occasionally the reply is "I need to find out" rather than a wild guess like some other manufacturers support people freely provide.
I have excellent support from some other suppliers, but that is because of "being friends with" someone at the company, SILabs is the only company I know of where the 'first line response' makes any sense. I "am friends with" some people at SILabs but see no reason whatsoever to not just send my queries to "general support"

Erik