Treatment serves two functions. It stops termites and it kills fungi which cause rot. There’s very little experience/history with the replacement treatments. I’m seeing something green growing on treated decking locally that was installed less than a year ago.
I’ve had some of the 2.4 pcf stuff in a similar application and it looks the same after 10+ years. In a freshwater environment, the 2.4 pcf treated lumber will be good for many decades.
If it was my house I would use concrete, IPE and PVC. None of these three will rot in our lifetime.
Would it be worth my effort to weather-proof my deck even if the wood looks like it’s old and dry-rotted?