When you think about it you realise that the totality of everything on planet earth is a constant (pretty much) so wherever it happens to be is neither here nor there. Landfill is just a focus of objects that otherwise were somewhere else but now aren't.
Pretty much, so far as the atoms are concerned (apart from helium), but it's the way they're joined together and co-located that matters to our future survival. We're taking useful molecules like fossil hydrocarbons and reforming them into toxins (among other things); there will never be any more fossil fuels however many geological ages there are, unless all life is wiped out and restarted, as coal and oil formed before fungi evolved. We're taking bulk minerals, extracting the metals and dispersing them. There are a few elements that are more concentrated in landfill than the ores we currently mine for them, but mining landfill has a whole new set of hazards.