Thunderbolt 3 is the way forward from now
...or, rather, it is the future in Apple's alternate reality (in which USB-A, DisplayPort and HDMI are already "legacy connectors") which Apple need to be true so that they can make laptops too thin to host HDMI or MiniDisplayPort sockets.
Back in the real world, Thunderbolt 3 has been around for a year or two now, and there is still only one Thunderbolt 3 display on the market - an Apple/LG joint effort. Even Apple/LG's smaller 4k display uses USB-C's DisplayPort alternate mode (i.e. DisplayPort physically running over a USB-C connector) rather than TB3 (virtual DisplayPort links tunnelled through TB3 protocol).
Thunderbolt 3 is limited to DisplayPort 1.2 and doesn't support DP 1.3 or 1.4 - so it still needs to use the MST kludge for 5k and larger displays.
In current PCs and Macs, most TB3 ports only output graphics from integrated GPUs or low-power mobile GPUs. In the PC desktop (and Hackintosh!) world, where serious graphics users & gamers use PCIe GPUs, there's absolutely no point in connecting the display via TB3 - which involves adding a TB3 card with an external cable going to a DP output on your graphics card - when all GPUs have HDMI and/or DisplayPort connections, possibly supporting newer DP/HDMI standards than can be used over TB3. Even with the internal graphics, the only reason to use TB3 for a display connection is if you're using multiple displays and you have a typical silly PC motherboard with 1 of everything, two of nothing port-wise. Even then, a USB-C to DisplayPort adapter does the job.
...and with the new 2016 MacBooks, with 2 or 4 TB3 ports being the
only connections for power, USB, display or storage, you're going to be reliant on a multiport adapter anyway, so may as well get one with DP or HDMI...
So, no - I don't see TB3 becoming dominant
on third-party displays for a good while. I think we'll see a mixed market: HDMI at the low/gaming end (compatible with consoles, BD players and set-top-boxes), DisplayPort at the higher end (pro graphics & power users). There may be a niche for USB-C or TB3 displays designed as docking stations for laptops and mobile devices - but then why go for TB3 instead of USB-C alt mode, and hence exclude most mobile devices?
...also, I get the impression that there is not much demand for 5k displays outside the Apple world (in which it's a "sweet spot" being a pixel-doubled version of the old 27" iMac/TB display/LED Cinema display).