No, DP 2.0 tunnelling support won't be added. It's not in the specification. And there is not enough bandwidth. As you wrote, DP 2.0 takes up to 80 Gb/s of bandwidth. Neither USB4, TB3, nor TB4 have that much. You can't tunnel 80 Gb/s worth of video over a 40 Gb/s duplex connection.

Also, DP tunnelling doesn't imply PCIe tunnelling. USB4 might have adopted the mechanism of tunnelling from Thunderbolt but the requirements aren't the same. USB4 requires DP and USB tunnelling support. Thunderbolt compatibility and PCIe tunnelling are both optional. Yes, if AMD wants to support USB4, it absolutely needs to support DP tunnelling (which is based on DP 1.4a). But it doesn't need to support PCIe tunnelling.

DP 1.4a is a good fit for TB3. It takes up to 32 Gb/s which is 80 % of the available bandwidth. You can't go much higher than that while keeping tunnelling of other protocols viable. Bandwidth taken up by a video signal is constant. It doesn't fluctuate like network or storage traffic. So, I think the benefit is quite limited even if you could support DP 2.0 only partially.

An interesting aspect is encoding. DP 1.4a uses 8/10 encoding (every 8 bits of data are encoded as 10 bits for signalling - it's a property of the physical layer that takes care of actually transmitting data). Which means that 25.92 Gb/s of video takes up 32.4 Gb/s of bandwidth. USB4 uses more efficient encoding, up to 128/132 (DP 2.0 also uses 128/132). The very same stream would take up only 26.73 Gb/s, saving almost 6 Gb/s. At that point, partial support for DP 2.0 would look more interesting. But as I wrote, it's not in the specification. AFAIK.