As I understand it, automatic fallback/limitation to HDMI 2.0 speeds was desired by VESA in the event of using an 18gbps cable or other signal integrity issue, so ultimately they chose to require the host to be more aware of HDMI for the converter to enable HDMI 2.1 speeds rather than requiring the converter to be smart.

Displayport to HDMI 2.1 adapters already seem quite complex: https://www.paradetech.com/products/ps196/

Is the problem expressing the maximum supported bandwidth end-to-end, i.e. not as a function of the DP channel quality that the source sees?

Yes, as a specific example, if the HDMI sink wants DSC, maximizing the quality (minimizing the compression ratio) fundamentally cannot be done without knowing the end-to-end bandwidth.