I agree w/you on the basis of this thread, Retiree (or should I call you, "old-timer"? LOL).
But I disagree, generally, altho I have little to go on, that horses today are the same as oh, the last part of the 1700s. I would posit they are essentially the same as in the late 1800s; before that, were too close to their half-breed roots to be as good. IOW, the pedigrees were not as concentrated so that they truly had a pure breed of TB. I think all 1700s horses were too close to the Arabs and other breeds to possibly compete w/those today.
BTW, I wasn't sure your proposal referring to the Arab stud. Every1 knows (or should know) that Arabs certainly were not the only breed used and certainly Arabs cannot simply morph into TBs w/o other breeds/mutts. It's just that (like males generally) Arabs were the only noted breed (outside barbs and turks, some of which may have all in fact been interchangeable) in the pedigrees - made worse by the fact many were Arab STALLIONS - and males get all the attention.
(I fear I mite look like I'm contradicting myself in this thread. Suffice to say I believe there is a possibility that horses are better today than in MOW's time or even in Big Cy's time, but I don't see solid proof, what w/all the confusion set in by the technological changes. Too many factors play in in the last century, even w/in that whole period. Of course BAD breeding can likewise make the horses worse - as many worry about, but I see no solid proof of that either.)
