I think if there was no share back requirement you would still have one project that would be the social nexus of the data. Forking and closing the database just to add some trails in Wyoming would not make much sense (as a silly example).

In essence, careful respect for the license isn't the main driver for contribution.

I think it would certainly lead to more competitors, people would take the data and do their own value add, but I think it is quite an open question whether they would be able to actually become considerably better databases.

(I'm roughly indifferent to the share back requirement. I don't think it is needed but given how much support there is for it in the community it isn't worth discussing over and over)