I’m a little surprised questions are still coming up about this, so for anyone who doesn’t know, here’s a quick summary.
Freesync isn’t just an “open platform”, it is a VESA standard. AMD developed it, opened it up, and sent it to everybody to use freely. VESA adopted it after.
FreeSync is royalty-free, free to use, and has no performance penalty. [Wikipedia, sourced]
It also doesn’t raise the cost of monitors. Monitor manufacturers bake the VESA spec into the hardware of their monitors themselves.
G-Sync is proprietary and requires hardware that typically adds $200 to the price of a monitor. Nvidia manufactures chips that monitor manufacturers have to use in order to support G-Sync.
Since FreeSync is a freely available standard, Nvidia could make all of their future GPUs compatible with all of the available FreeSync monitors on the market. Whether or not they could do this simply with a driver change for existing cards I don’t think has been directly answered by Nvidia and I imagine they are the only ones who would know for sure, but maybe. AMD can never make G-Sync capable cards because Nvidia didn’t share their tech like AMD did. No judgment here about that, that was Nvidia’s business decision to make. As for not supporting FreeSync, I do judge a little about that. :P