Thanks for pasting the chat log, that sheds more light onto things :)
Seems to me the spec author(s?) are not sure themselves what would be the best naming pattern (besides rtld namespacing) and are still sorting that out.
Which to me leaves the choice between being consistent with existing naming practice (at least in KDE spheres) and going for some unclear motivated naming recommendation not yet hard-coded in spec policy. Where I personally then would tend to the former, leaving the .desktop suffix.

But no hard opinion, leaving the call to the maintainers.

Grepping across KDE's sources with https://lxr.kde.org/search?_filestring=&_string=launchable&_casesensitive=1 it seems that adding <launchable> needs more advertizing in general. Do I correctly assume that Discover and friends already make use of that data bit, so it's valuable to have it set? Would then go and add for apps I care a bit :)