I don't see anything wrong with it, an alternative venue for playing on when its pouring with rain out.
Rays MTB park in the US is very popular.
Ooo just seen its planned for Greater Manchester area as well. Would be up for a visit if it went ahead. Not on my retro bike though
Indoor dirt I is not new and is good for people that want to ride in all weathers. Because of the compensation culture at the moment more and more trails are being flattened and closed so it good that people who want to ride have an alternative. If you don't like the idea just don't go
And just about every trail ridden is man made! :facepalm: