I have seen adverts for sort of plastic socks that you can secure around wooden posts at the point where the surface of the wood air and earth all join. Cos as you say this is where they give up the ghost.The most obvious example is gateposts. They always go at around the ground level where the microbes and fungi can make a meal of them. The bottoms are three feet down or so, and are often very well preserved, complete with tool marks. Even if they're sitting in the ground water for months at a time. When I put my fancy new gates in on the smallholding I dug up about three or four. Not bad timber either - I've been using it for firewood.
I have replaced, or paid to have replaced,sooo many posts along our fence line, no matter that the timber was tanalised.
Last time, we had the stumpy concrete posts put in to which the wooden ones are attached above ground. Not pretty but practical.