There is a practical problem that VirtualBox is in Debian contrib and not main because it cannot be built with tools in Debian main. (Whether this is a practical problem that matters to anyone is a different question, but it's more than theoretical.)
I don't think DMC is in Debian main yet, is it?
Probably not yet, but the license means that it can be.
So that is also just a theoretical idea at this point.
Why should VirtualBox take the pain now rather than waiting and seeing if the theoretical issue every materializes? I mean, if someone does the work and tests it, then sure, VirtualBox should adopt it. But why is this their problem to solve now?