Thank you qwerty13579! How stupid of me, the solution is obvious. I have tried all sorts of things suggested over the internet, and even with the most recent version of VS Community 2017 (15.9.9), the bootstrapper failure pops up erratically, but with increasing frequency, to the point of frustration this past week. The principle: It doesn't succeed reliably, so it also doesn't FAIL reliably. The solution: Click the Publish Now button and watch the output panel. When it's that clear that it's failing, click the Publish Now button again immediately. Keep at it until it succeeds! Each round only takes a couple of seconds, much less time that it takes to re-build, take down anti-virus, and all that fancy stuff.
In my case, I traced the problem to an incorrectly dated setup.exe file in the bin\Release\app.publish folder of my application. When it fails, Publish Now creates an setup.exe that's two years earlier than the current day. When it succeeds, the setup.exe file is correctly dated.