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It does seem like a credible case can be made that the release of Stretch was rushed. (For the record, I thought the same thing about Jessie.) Really old-time Debian users remember fondly the days of "it'll be released when it's ready!" In contrast, for the last few releases, RC bugs have been ignored or arbitrarily reclassified in order to meet a specific release date.
Sometimes, there was a promise that the bug(s) would be fixed in a subsequent point release. Sadly, that promise often proved to be worthless.
As regards this particular bug, for a system to be unadministerable (is that even a word?) absolutely qualifies as a show-stopper. Ipso facto the release of Stretch was premature (by exactly how much being the only remaining point of contention).
It's sad but not surprising that this bug affected the installer. As long as I've been using Debian, the installer devs seem deeply resistant to change and actively antagonistic to constructive feedback. When I first tried Debian, I found myself thinking that the installer was by far the "weak link" in the entire UX. Here we are many years later, and I haven't found a reason to change my mind.
In fact, as I think about it, the installer is home to many if not most of the "we'll fix it later in a point release" RC bugs mentioned above.
Perhaps the release manager for Buster will take a more enlightened approach to its release. (But I'm not holding my breath.)