A failsafe works when your contrroller loses power or when signal is lost because of too much distance right?

But it cannot do anything when the reciver battery is out of power or gets disconnected.. In this case what is the solution?

Not positive, but I think if it's a nitro vehicle you can have a throttle return spring. Seems the spring would need to be strong enough to overcome a dead servo, but not so strong as to overwork a live one.

If it's electric, about all I can think of is developing a close relationship with a Higher Power!

But more seriously, I'm not sure, I don't use a receiver batt, but I think the speedo would shut down the motor if it lost signal from the receiver.

Usually when the battery fails or falls out of an electric car the car would kind of stop by itself. No juice no go. Which is why failsafes are not used in electrics. As far as interference or out of range issues are concerned, most if not are ESCs stop sending current to the motor. BTW some very old mechanical speed controls (wiper style operated by a servo) could benefit from a failsafe and TRS.

With nitros the failsafe will not do anything if the receiver pack fails. The only solution is a TRS as stated by DaveH.