Trails are there to be ridden whatever the weather 🙂 . The best ones, at least the ones outside of trail centre trails, are constantly evolving and are different every time you ride. The amount of erosion is not as damaging as claimed, and nature has a great way of recovering when conditions improve. The only problem is with people not riding through the centre of muddy pools but going around them as you get trail creep and damage to the edges, plus it turns sweet singletrack into dull doubletrack.
Don’t think I could stand the US rule of staying off trails in the wet in case of damage. We’d never ride! 😀
Besides if we didn’t ride them the ramblers would stomp all over them and the damage would be worse 😉