All them mountains and tracks in the lake district/peaks/south downs/scotland etc certainly weren’t designed with any wheel size in mind. I don’t think there’s any need to start worrying if you ride a 26″ wheelend bike. I won’t.
This is the main reason why some manufacturers are still selling 26ers, because it’s too labour intensive to upgrade the natural trails to suit the larger wheel sizes.
Another thing is some of the more popular trails have put up signage next to features that are deemed not suitable for 26ers, with 26er chicken runs off to the side.
I’ve also heard that some trail centres are considering setting up 26er specific parking which is closer to the trail-head and, in some cases, futher along the trails to compensate for the extra pedaling diastance 26ers cover. Madness, all this expense, must be getting some serious back handers from the manufacturers.