I imagine most people never replace their bath tubs because what can go wrong? Most bath tubs don't suddenly break apart, unless, of course, you are unfortunate enough to have one of those fiberglass or acrylic bath tubs. Especially those one-piece tubs with seams that can separate. Or, you live in Hawaii.
Many people in Hawaii remove their tubs completely and install showers. Others, if the finish chips because they've dropped, say, an anvil in the tub, well, you can get your tub refinished. It's more cost effective to refinish the tub than try to remove a heavy cast iron tub and replace it. But I have discovered there are cast iron bath tubs, and then there are good cast iron bath tubs.
First you have to decide what type of tub. Its configuration style, and there are many. You can read more in my personal blog today at this link: My Adventure of Buying a New Bath Tub in Hawaii.