If a canvas is sagging are canvas keys the only option?
No there are options.

Being able to use canvas keys depends very much on having the right type of stretcher bars to work with.

There are alternatives to help tighten a sagging canvas.

One method to help with sagging canvases is to spray the back surface of the canvas with a light spray of water. As the water evaporates the moistened canvas fibres will begin to constrict.