The theme switch was anecdotally reported by users, and I have been given firm reassurances that that change was completely unrelated to the HTTP errors that prevented what seemed like a sizable portion, maybe even the majority, of the userbase, from being able to log in or otherwise interact with the forum. For that reason, the initial announcement was soon amended, and that piece of advice removed: while essentially harmless, it served no real purpose and a had a potential for confusion.
A number of changes had been made before that weekend, so it may seem logical to draw a connection between any one of them and the problems that ensued; the real cause was never disclosed, AFAIK.
It’s true that, after the resolution of the 403 errors, admins stopped being active almost entirely, unfortunately that’s more the norm, rather than an exception.
Outside of whatever personal preferences there could be about any individual forum software, there were good reasons for upgrading vB4.
Probably the most important was vB4’s old age and its being in “long term support” (I forget the exact denomination), only receiving patches for outstanding vulnerabilities. For any other issue, there would be no fix: remember the problem with the editor not working correctly for a long time, when using the Reply With Quote option?
The original announcement maintained the forum would be upgraded to vB5, however, that decision was later changed; I don’t remember whether Graham said anything about vB5 (he did comment on some of the alternatives, just search this thread), I doubt that was just a last minute change of mind.
I can imagine some reasons that could have tipped the scale in favor of Discourse, those would be mostly my speculation, though.