Depends how you define “trail center” though doesn’t it?
On the one hand a former roman road on the yorkshire moors isn’t ‘natural’. But neither is a bit of Singletrack at Tunnel Hill (but it it rooty/bumpy/twisty/all things good). You could formalise Tunnel hill just like Swinley was (if the army wasn’t there).
Basicly, it’s all good fun, and frequently the ‘trail center’ is more fun. The USP of ‘natural’ rides to me isn’t that they’re better riding (frequently they’re crap, boggy, too flat, too rocky, too steep, or overgrown), it’s the getting ‘out there’ or exploring a new route, seeing a view, acomplishing a challenge. Having said that I don’t travel very much for MTBing, I ususaly just ride whatevers on my doorstep at the time.