There are many things for which there is no substitute. Your mother’s home cooked meatloaf is, in memory, and in fact, one of a kind. There is also no proxy for the foods of your childhood, either. In truth, even if you remember it ever so fondly, the stews you savored in your youth may not suit your grown-up palate. Tastes change. Times change.
The seemingly irreplacible can find a substitute in something newer, more modern, more up-to-date. The replacement you find may be just fine.
On the other hand, a replacement may prove inferior. It may be unsatisfactory and unsatisfying. You face the risk that a swap is a poor stand-in for the original. If that happens, you may want to replace the old for the new.