Is it possible to restore original firmware? (Asus RT-AC68U)

Thinking of getting me a netgear nighthawk or an Asus RT-AC68U and was wondering once I flash the firmware with OpenWRT and say I want to sell the router in the future, its possible to revert back to original firmware or this depends on the specific router?

I read somewhere that Asus routers are hard to brick so my guess is yes. Just TFTP the original image.
Question is, why would you want to ?