500px is an art site which has dipped its toes in photo stock sales. The images on 500px have a different feel than the majority of microstock and that is attractive to buyers for some purposes.
I think that if 500px were to meet the oft-stated desire on MSG for proper watermarks, it would simply open the floodgates for a lot of contributors to dump large portfolios of images they already have on other microstock sites, in the hope of being lucky enough getting a good priced sale or two. The ingestion of such a body of imagery would change the nature and feel of 500px and it would, eventually, become just another microstock site competing in the same market. The alternative would be for 500px to ruthlessly curate the incoming imagery to maintain the look and feel of the collection.
On the whole - and I haven't as yet decided whether to sell my own images on 500px Prime - I would prefer things at 500px to stay as they are and for contributors to make their own minds up about whether it is an appropriate seller for their images. We don't really need yet another microstock agency.