As a sysadmin I object to the notion that old to new conversions are a "fringe" use-case. Pretty much all my coding (I'm a "devops" sysadmin-type) is exclusively "I need to wrangle all versions of thing 1 into a type of thing 2 or between several versions of thing 1".

If backwards compatibility is important, then letting me use multiple versions side-by-side in the same modules easily is damn important.