Remember that FreeSync can also work through HDMI now, so it doesn't even need to be changed to DisplayPort.
I do find it strange that HDMI is used on TVs, given that it is a royalty based standard. DP is Royalty free if they are a VESA member.
All FreeSync/VESA Adaptive sync needs to work is a hardware scaler in the monitor that is compatible. This is certainly something that companies should be considering moving forward, especially since they already need to upgrade for HDR support.
Samsung is already supporting FreeSync on most of their new displays, so FreeSync on their future Televisions is a possibility. It is just the chicken and egg scenario that prevents this from being a thing.