You wrote that reasonably smart people couldn't figure out the old system, even with a couple of days' investigation. How is this a good system? If people can't figure it out, how are they supposed to use it?

Then you wrote that, when the new system appeared, "nearly everyone" moved to it. Doesn't this suggest that the new system was actually better? Why would most of the old system's clients, for example, pay the price to move to the new system unless they believed it was improved enough to be worth the migration cost?

I'm not saying that your conclusions are wrong, just that from reading your anecdote, it's not clear how to reconcile your claim that the rewrite was an mistake with the evidence that the result of that mistake seems to be preferred by nearly everyone who had the choice to keep using the old system.