I think that many professional may have already jumped ship.
$2,000 Custom PC vs $4,000 Mac Pro
Given so much of the Mac Pro's market is professionals, the professional think for Apple to do is either update it or be honest is their loyal users that the Mac Pro is dead and announce that it will be discontinued on some date. The Mac Pro isn't like that iPhone, Apple don't need to be so secretive about it. When Apple transitioned form OS9 to OSX, they were open with people about the time line, they allowed professionals to keep running OS9 for sometime if they needed to.
It's not the hardware that would make "jumping ship" so onerous, it would be getting used to the awful OS that is Windows. Many video editors have left after the FCP fiasco, but they went to Premiere for Mac. And many of them are cutting on 5K iMacs. I'm talking about professionals in broadcast shops using iMacs. That's still not something I want to do, as I like server grade hardware and my 2012 Mac Pro Tower is still chugging along. I'm hoping they update the trash can Mac Pro, but it may be on a 3 or 4 year cycle now. EVERYTHING else in this long thread is speculation and you can basically take many of the posts and insert a few different names and they're exactly like posts that were put up 10 years ago. Let's just see what happens on the 27th and skip all the bloviation and drama until then.