Comment # 6 on bug 1205009 from Stefan Hundhammer

When you use that list of old repos for the purpose of editing the URLs to
point to the newer release, then I do see a valid use case for this.

But in the more general case, this might result in an accident waiting to
happen: When we offer to keep the old repos, and they also keep their priority
(probably by default), not-so-advanced users may be tempted to just keep them
and then NOT change the URLs to point to the newer release (because they don't
know that that's what they should do).

And then they will end up with a bunch of repos with higher priority (because
by default we add the new repos with a very low priority, so any other priority
will always be higher), so they'll get packages from those old repos pointing
to the old release, and weird things will start to happen.

A few very advanced users will know how to make good use of such a feature:
Just keep their old repos, keep their priorities, and edit each of those URLs
to point to the new release. For them, that would indeed be a handy shortcut to
save some work.