I'm seeing another now in $current_job and I'm seeing similar symptoms (though the system being rewritten is far less important) and customers of the old system essentially abandoned to themselves and marketing and sales are scrambling to try to retain them.
Anecdotal experience is not so good. Rewriting a tiny component? Ok. Full on rewrite of a big system? I feel it's a bad idea and the wisdom holds true.
I guess that in your case probably (1) did not hold. Or maybe (2) did not hold, or both.
OP's experiment doesn't prove at all, that an entire org can rewrite a complex app where 1&2 do not hold. Every indication we have is that org's executive functions perform abysmally for code writing (and rewriting). So exactly the point you are making. It would obviously mean that there is value in code, along the value in the org, once we get above the level of the value that conceptually fits into 1 head.
...you're gonna get bad code again, or, as you say, worse. The impact of the organizational culture dwarfs everything else.