Sure, and there's 100's of thousands of replaced bridges that don't immediately spring to mind.

Though to be clear designed to last 50 years isn't the same as saying it will only last exactly 50 years, or that theirs nothing you can do to extend a bridges lifespan if it's replacement is running a little late.

If it is a famous bridge, it seems possible that they’ve done some special extra maintenance, replace more wear-and-tear bits that would normally fail. Eventually you probably get the tourist attraction of Theseus.