I ran for over 4 years and didn't label my soil bins and ended up with various unknown batches, but I was able to easily compare. What I discovered is that 3rd, 4th, etc run soil will be much blacker and denser that 1st run. I recycled all my foliage and roots as well as heavy additions of EWC during recharge. That adds a LOT of humus over time. Mixing in fresh ProMix restores the balance. But humus should be a good thing, increasing CEC. I think the problem is the biota populations and the byproducts of their work. We get increased mineral conversion, but also increased salts, and they eventually turn the soil sour. Cutting with more peat also solves that issue.
Ideally, we want to keep the humus but get rid of the salts. Flushing would seem to be the best solution.
I also wonder if a long resting cook would do any good.
I have some bins I haven't opened in a couple years, but I don't see how the salts would get converted in that scenario. Nature takes care of this with constant flushing and percolation.
So ... dilute or flush. Well-managed 3rd run soil can be amazing!