We’ve been sleeping on Queen size koala for 3 years and it feels fine to me. Only 3 years in so can’t speak to the longevity. I still feel comfortable. Mattress feels good. Does sagging depend on the build quality of your bed/slats as well?
Since we’re happy with it, we just bought a single size koala for our toddler to move her out of the cot and into a real bed. She’s sleeping just fine… And through the night for the first time in 9 months.