Monetary cost is the wrong y-axis here, as it optimkzes only for mega-scale farming without taking its real costs in consideration. It should be ‘true cost’, which also accounts for environmental-, animal- and climate mitigation cost.
Legumes tend to be very filling for the amount of protein they provide, plus having to complement them with grains to make a complete protein greatly increases the Calories to protein ratio. Rice and beans do make for a great cheat meal when you can fit it in though.
Incorrect. Legumes do not need to be complemented by grains. Some grains like wheat may need to be complemented by legumes or some other protein source however, though this is not true of all grains, such as quinoa.
They can, but it takes a large amount more work to balance for complete amino acid profiles in the diet.
Meat and eggs are "complete" proteins, which contain all the aminos you need to build muscle. Plant proteins are "incomplete" proteins, which means the amino profile is not complete and missing some amino acids.
It's not insurmountable, but some planning is needed to fill in amino gaps in one incomplete protein source with a complementary protein source. Example would be rice and beans tend to be complementary.
Your post contains a lot of incorrect information. First of all, basically all plant proteins contain every essential amino acid. You can nitpick about certain foods being low in certain amino acids but void of them they are not which is what you are claiming. But also your statement on complementary foods is incorrect. Legumes and most vegetables don't need to be complemented by anything. Certain grains like wheat are lacking, but quinoa is fine. Potatoes are lacking, but squashes are fine. Basically all fruits are lacking and just low in protein overall. "Don't eat a diet of fruit and bread." is a pretty big step from "It takes a large amount more work..."
And this is tangential but we should be concerned about nutritional content in general, not simply the amino acid profile. Amino acids are simply just some of the many essential things our bodies need. Compared to vegetables like spinach, steak and eggs are by far more nutritionally incomplete.