By “single source” do you mean “single stream”? With “single stream recycling” meaning consumers put all recyclable products into a single bin, vs. multi stream, where consumers are expected to separate recycled goods into multiple categories.
If so, then yeah, multi stream is generally considered more efficient. Obviously some central sorting/filtering is still needed, but much less.
As for recycling in general, it’s considered a decent win for plenty of materials. Making cans from recycled aluminum takes only a tiny fraction of the energy of producing new aluminum cans, paper recycling does reduce the need for forestry, most recycling reduces the need for landfills, etc. It’s mostly plastic recycling that is useless, and just there to make ppl feel better about their plastic consumption.
It wasn't so much "science was settled" as much as the plastic industry lobbying to improve their image and put the burden on consumers to "recycle" plastics, despite knowing it wasn't going to help things. And unlike conspiracy theories -- we have evidence.
many plastics are good fuel; but it's been difficult to say so for years because the "science was settled" that recycling was The Way.