World class xc riders generally use hardtails at the moment, I can see a point where they mainly use full suss as a preference at some time, but it may not be in the forms currently popular.
There would be 0 benefit to full suss as currently known on the TDF.
Any springiness and dampening required can be engineered into the rigid bikes.
It is all down to the size of the obstacles.
Someone mentioned cobbled roads, well I will totally agree on most rigid forks these are a pita, but try them on a Jones.
There is nothing about 'suspension' that some clever steel working cannot achieve, albeit with vastly different geometry.
When you ride a hardtail out of the saddle you are using the front suspension and your legs as full suspension, everything going forward rotates around the front contact point.
You load that up, and the rear is trailing.
Rear suspension generally only becomes efficient when seated.
Modern or not.
Standing makes me a dynamic and responsive machine, sitting makes the terrain dictate most of the time.
Getting back to the thing about the New Forest, I would have to say it is one of the most beautiful places I have ever been and whatever reason takes you there please go and experience it, to prove a point or not.