There is room for improvement but I'm pretty ok with modern landfills. Methane capture IMO is the biggest lack. It's such a waste to let it pollute the atmosphere (at 30x the GHG potential of CO2) when you can capture it, scrub it and burn it for energy. Google says landfill gases account for about 10% of global GHG emissions.
(That's in North America, probably most of the world has woefully inadequate waste management solutions.)
It's clear there's not enough thought and resources going into preventing environmental problems but IMO landfills are not at the top of the list.
> I've recently move to the Netherlands from the UK and was amazed to discover they don't recycle household metal items
Wikipedia says metal cans are separated from regular trash (at the plant) to be recycled. It also says recycling in handled by municipalities so YMMV.