In practice, however, what typically happens here is that you have your killer feature you want to sell, but will find other things you want to fix. You don't want to maintain all of your fixes yourself, so you try to modularize your enhancement off, and contribute everything else back.

Failure to do this means you end up with high maintenance costs if you want to pull in enhancements from future versions. So the fragmentation tendency is only partial. You have to either fully go your own way (usually suicide) or you have to give almost everything back (which opens up the possibility of competing with the community).

Sure people may choose unwisely. They will suffer the consequences and maybe learn lessons the hard way. ;-)