Old strawberry plants

I have some old strawberry plants over 5 years old, still growing strongly and looking healthy and robust with 2 or 3 large crowns each. I've read that after 3 or 4 years one should dig up old plants and replace with new ones. But if the old ones are still growing away healthily, is there any point in replacing them?

Yes replace as the yields will dwindle as the plant gets older.
You can keep the same strain of plant by taking the runners and popping them into pots and let them root before cutting off from the parent plant. You can then plant them but I would suggest in a different place to lower the risk of disease.