Last week I installed two machines with Tumbleweed and Leap 42.1, respectively,
and in both cases I chose an LVM/LUKS-encrypted installation using ext4 and no
separate /home partition. The installation process worked fine. However, in
both cases the installer created an install plan that reserved only 10 GB of
space for the system's (encrypted) root partition -- leaving some ~450 GB of
available disk space unused.

That choice seems odd to me. I realize that it's a good idea to have some free
space available on an LVM-managed disk so that people can create additional
partitions, performs snapshots, etc. From that point of view, I think it's fine
that the root partition didn't take up *all* the remaining disk space despite
the fact that I chose the "use the entire disk" option. However, allocating
only 10 GB for the root disk seems way too conservative, and I reckon that many
users will not be happy with that default (like myself).

My suggestion would be to implement behavior in the installer that works the
other way round: rather than allocating a default partition that's minimal and
trust that people will enlarge it when required, I'd rather see the installer
create a root partition that's maximal and trust that people will shrink it if
they want additional volumes in the /system group. In any case, 10GB for the
root disk seems like an odd choice that's not going to work out for virtually
anyone.