When to replace a underlying mattress under 4 inch topper

I have a Topper question:
For over 10 years , I have successfully been using a strategy with putting a 4 inch 5lb topper over a very firm inner spring mattress (Simmons beauty rest extra firm). I replace the topper every 3-4 years to a new one, which works well, but the base innerspring mattress is the same. My question is whether there is any need to replace the inner spring mattress now that it is over 10 years old, nearing 15 years. When I inspect the mattress without the topper, it is firm as ever, with no sagging or signs of spring wear that I can tell. So I’m not sure if there is any need to replace the mattress since I replace the topper which is what makes the difference.
Even to get a similar basic firm innerspring mattress as a replacement would cost around $500, so I’m not sure I want to do that unless there is a good reason.
-thanks, Javed

Hi Bostonsnow,

As long as your mattress is straight and is not sagging then you don’t need to buy a new mattress. I suggest you inspect it every 6 months or so when you do your rotation to see any signs of breakdown.

All the best,