Then you've used energy to lift the boom and presumably rotate the machine.
Then you've used energy to "tip" the bucket so that the material falls out into the desired location.
At what point are you going to be recouping energy?
Practically they rarely are, because the way a piston is moved 'slowly' is to have a half-open valve with a pressure drop across it, and that half open valve wastes all the energy into heat in the fluid.
Plus as you say, excavators don’t use a winch and motor, they use hydraulics, and I don’t think the former would be powerful enough to move lots of earth. But OP said “theoretically”, which makes them technically correct (the best kind of correct.)
There are proposed energy storage systems that involve simply lifting a lot of heavy stuff to store energy, and then lowering it again to recover it. So the idea isn’t even new. But said systems have terrible energy density (lifting and lowering stuff doesn’t store very much energy overall) and are basically a terrible idea as it is.