I think it is a great idea for trail centres but it doesn't go nearly far enough.
What we need is a system of ropes and safety harnesses so that when a potentially dangerous section is reached, riders can hook onto the rope and if they fall they could be saved from a nasty injury. It would enable entire families to go down 'blacks'. Dad or indeed Mum, can show how rad they are by descending 'unhooked' safe in the knowledge that their offspring can follow in total safety. Less confident riders could also try a dodgy section fearlessly, safe in the knowledge that the worst that can happen is bruised genitalia from an improperly positioned harness.
Alternatively, why not save money on those expensive mtb's and simply tour the theme parks for a thrill. You will get plenty exercise walking from ride to ride and have none of those creepy crawlies to deal with or be surprised by a rock which someone thoughlessly forgot to remove from the trail when it was carved over the last few centuries. You will also be able to graze on a variety of 'proper' food from the many fast food establishments dotted around thus negating the need to carry cumbersome backpacks.
There, all sorted 😉