What came to mind when I read this is a question: how are we getting more efficiency out of eating plants than ruminants are, when they are animals that have expressly developed over eons to do so?

We aren't, that's just a general trend in biology. On average, consumers only receive about 10% of the energy that goes into growing the organism they eat. To be clear, I'm not saying the consumer is getting on 10% of the nutrition they eat, but that the amount of energy in the food when it's eaten is only about 10% of the total energy that organism needed to grow.

You shouldn't belittle people on the internet, especially since you don't know me very well at all. But we are civil here, so I'm going to let that pass :)

That's a fair call-out, I'll do better.

Even considering usage of pesticides, chemicals, and the loss of topsoil and biodiversity? It is estimated that we only have 90 harvests or so remaining. What happens then?

All of these things are worse with farming animals. 60% of the cropland in the US goes to feeding cattle. Those cattle proved a tenth of the nutrition that that cropland could produce otherwise. Eliminating cattle farming would double our available cropland and make regenerative farming practices actually possible.

To be clear, I certainly don't think cattle farming needs to be eliminated, it's just not feasible to be such a massive part of our diet.