I just love the idea of a Earth in the far future, where the huge pile of plastic rubbish left by humans is rotting like an old log.

We might have inadvertently created an enormous pile of fertilizer for a new world :-)

Right but our entire infrastructure has been built to resist that. Most likely it'll happen at a slow enough pace that we could adapt, but just imagine if soil fungi could suddenly eat PVC pipes

Most traditional architectural techniques didn't try to resist rot so much as plan around it. I.e. have the structural integrity so that you could take out and replace parts of the building without the whole thing coming down. It's why Japanese temples are the oldest standing buildings around today

I imagine we'll have to relearn some of those lessons at some point

I think a lot about this. How on a geological timescale, most of our rubbish will be compressed down to a layer of oil with a bunch of increasingly heavy metals and radioactive isotopes towards the bottom. It'll be a rich vein for building a civilization for whoever comes along in half a billion years. (My bet is on the octopus).

Wonder if it'll degrade to something oil like? :)