I do agree with the part regarding antibiotics will decrease the whole body's stress from bacteria/ bacterial load, allowing our body to cope with the infections. I do believe in the hypothesis that below certain bacterial load, our body will be able to clear the infection, but once above the threshold level, we will lose the war by ourselves.

However, regarding the "full" course, it is vaguely described, if a doctor prescribe a two days course (which in the past might be 10 days), your post become misleading!! Furthermore there are actually studies talking about sub-lethal dose of antibiotics could stimulate bacterial virulence genes, which makes them more aggressive to human, that likely meant more toxin secreted and more adhesion protein. Increase in virulence is not favorable to the infected host. @mcw had a post briefly described this HERE

Also for S. pyrogenes, not having develop resistance to Beta-Lactam might be the result of its small genomic size, mutations or evolution of resistance genes are always more favorable if you have an extra copy of the existence gene, so that the bacteria doesn't die by losing the only gene when their mutation might lead to a non-functional gene.

I will still recommend people to finish the prescribed dose, after all many of us are not doctors, and failing to take appropriate treatments, and if we miss the golden treatment window, it is always us the sufferer