Not to mention that the dynamic would completely shift in terms of community contributions. If I submit a patch to a free project where the maintainers make nothing, I wouldn't even think of asking for anything in return (even if it is a project used by bigcorps, such as Redis or GHC). If I know that the maintainers get paid a full salary for maintaining the software, it becomes a much weirder thing to send them bugfixes for free.