There are commercial extensions and services out there that do this (e.g. Merchantry or uDropship, uMarketplace), and you can manage external marketplaces from within Magento via services like ChannelAdvisor or M2EPro.
I have often said that the value-add is to bake in this kind of functionality but in reality this shrinks the marketplace of vendors down to trivial offerings.
That said, I do think this is an Enterprise-only feature at scale. You can get by in CE with existing commercial offerings; assuming that those offerings all upgrade their products to Magento 2 in a timely fashion.
Otherwise, would it really be so bad to be "stuck" on Magento 1 for some time?
Because this is already being handled by the community of extensions I think that Magento isn't in a terrible rush to provide support in-platform.