Gee. So many to choose from.
How about Einstein's theory that gravity/mass bends light?
It took several years, but that theory was absolutely proven.
No theory is ever proven
Har!!
How about Einstein's theory that gravity/mass bends light?
Light being bent in a gravity field is not a theory but an observable, measurable fact.
Einstein's theory on why it happens is the currently accepted explanation, and probably mostly true. That does not rule out a better explanation being found in the future.
What to do with relativity then, and the evidence that supports it, if Newtons theories of motion had already been “proven”?
It is obvious you have no knowledge of or respect for the methodology of science; but in science no theory is ever “proven”, just accepted provisionally awaiting contradictory data or refinement.