I don't think so. Landfills are a source of many important minerals and energy. I'm convinced that we'll be mining ours in the next couple of centuries.
Maybe, but if so, it will be because we have really substantially depleted primary mines.
A precursor civilization which needed to mine its landfills would therefore undoubtedly have left a visible scar in the fossil record. If they suffered some calamity before getting to that point, then we get landfillite. Either way, I think we'd see evidence of any civilization which sustained the equivalent of a 19th and 20th century.
Not necessarily just due to depletion - could be just better technology (eq. some massive robotics deployment, gassification & atomic sorting, etc.) that makes it a no-brainer to process all regular waste & the old ones when you are at it.
Even modern waste incineration comming quite close with iron & even alluminium collected from the slag & the mostly inert reminder reused in consturction. The local waste incineration plant already covers all district heat requirements of the 400k people city in the summer and is poised to cover 50% of the inter load once their new nr. 3 furnace is installed.
