Does anyone own a firm mattress that hasn't dipped and sagged over time?

[ImEasyLikeSundayMorning]

I got the Simba mattress with my blue light discount and it was so firm, my back stopped being in pain in the mornings - bliss!

But 6 months on, despite regularly turning it as per the instructions, it has dipped where I sleep.
I can rotate it and it's ok for 1-2 nights but then the dip returns and so has my back pain.

I can't afford a new mattress but I need to start saving for one.

Does anyone have a firm mattress that stayed firm?

--
MNHQ Update
If you’ve landed here while on the hunt for similar mattress recommendations, our guide to the best mattresses has been recently updated to reflect current availability and trends. It features a wide range of options and brands all rated by Mumsnetters. We hope it’s helpful!

Ours is an Otti, I think. Hasn’t sagged in the 5 years we’ve had it.

I had a Rem-Fit - didn't dip AT ALL - fab mattress. I have a Hypnos now and it's fairing very well too.

Same . Hypnos eight years on now and still great

Could you have used the 200 night sleep guarantee to return it? A bit late now I know… 🙁

I had the same problem with my Simba mattress. Lovely for the first month or two, before I began sleeping in a hole - despite regular turning of the mattress. My back was in agony, I felt like I’d been sleeping face down in a hammock!

Now bought an igel 3000i advance from Bensons for Beds, which has 3000 full individual pocket springs with 5cm of gel as a top comfort layer. It’s the most comfortable bed ever and shows no signs of sagging. Expensive, but worth it considering we spend 1/3rd of our lives asleep!

We bought a 3000 pocket sprung Luna mattress from a company called happy beds, initially as a bit of a stop gap when we first moved but it's great! No turning, rotating and no dipping or sagging. Not very expensive and still going strong 5 years in. I think the higher the pocket spring count the firmer it will be.