OK, after talking it over with Jamie, he’s going to try to implement some support for netplan into Webmin over the next couple of weeks. So, I think I’m not going to add reverting to the old way into the installer (messing with peoples networking in an automated fashion isn’t something I’m really comfortable with, anyway, as it could potentially disconnect the network session and maybe even break networking, if I got something wrong in the conversion, which seems likely with all of the pieces and how complex some of them can be).
So, for the time being, to install on Ubuntu 18.04 you’ll need to revert to the old way before running install.sh. Seems like some VMs already have it this way (since this is the first report of this problem I’ve seen and there have been a few other installations on 18.04 by other users who didn’t run into this). And, it may even make sense to do so on servers, regardless of whether Virtualmin and Webmin support the new way. Certainly the benefits of NetworkManager/networkd/netpland are most obvious in laptops, where networks change regularly due to mobility and maybe based on which user is logged in.