This is a sensible idea in principle, and has been discussed on various previous threads. I think it comes down to how do we determine if sources were provided or not?
Unfortunately there is as yet no useful automated software that does this for Wikitree. Until that becomes available, it would need person(s) to look at the profiles and make this assessment. I think that would need to have official Wikitree staff at least involved, and probably more, so would need to be balanced by staffing issues. Especially if applied to all new profiles.
I suspect also that Wikitree is balancing a strategic need to grow quickly and bring in new users, in order to not be overwhelmed by other players in this one-tree area. So they don't want to discourage newbies.
A possible approach would be to allow anyone to enter a certain number of profiles easily (as it is now). Say 100. (They could be allowed to edit any existing profile without limit.) Enough to have a useful tree posted here and an interest in Wikitree success. But in order to add more, they would need to demonstrate some knowledge of sourcing. Would be more than Pre-1700 certification but less than Pre-1500. Would certainly need staffing/volunteers, but possibly the number of people wanting to cross this threshold would be more manageable.