> The most blatant differences are surely in things like packaging and
> update delivery, but in a sense, these things are what you rely on the
> vendor for
Yes, but that's a huge difference on it's own! The next big difference is that the bits coming from those updates are very different.
> But anyway, the deployment of different distributions shouldn't be
> radically different
I guess this depends on your definition of "radically different". Some of the components are the same or similar (say, the version of vim, coreutils or maybe perl). But a _lot_ is different.
Configuring network is radically different between debian/ubuntu and Fedora/RHEL, a bunch of other configuration is almost as bad. As you said, all the package tool are named differently, act differently and have different feature sets. Packages are named differently, split differently
and often go into different places. Things like multi-minor-versioned python are std. in debian but not in Fedora.
If you hit a kernel bug in one distro. there's no guarantee it'll be in the other one (even with 10.4 vs. 6, the backport work is _huge_ on the 6 side ... and there are a bunch of changes from debian in 10.4). If it is in both you'll need to open two support tickets, and they'll almost certainly have different resolution timelines etc.
All of which is why I said it'd be nearly double the work, to support both.
> it's certainly not in the spirit of giving end-users the choice, which
> is what many people see (perhaps implicitly) in Linux.
There is choice ... you just have to choose given a number of differences.
Even if you forked from RHEL-6 beta tomorrow, by the time RHEL-6 came out you are guaranteed to have "more than one difference". If the user liked some differences on one side and some on the other, they'd have to choose the least worst.