BTW, something that I*think has not been said yet...

In my opinion Mer and Diablo are not necessarily two alternatives. I think it should not be difficult to build a version of Mer that includes the old libraries of Diablo and is binary compatible with Diablo and maybe even has the libraries of Fremantle only as an option. The choice Matchmox/Fvwm/whatever can be made after the installation of the OS even without restart of the OS. If the user selects Matchbox, the result would be an improved Diablo with possibility to install the software both for Diablo and Fremantle.