Actually, the American public schools have never been all that good. They were never, at any time, bastions of academic excellence.
A major reason they are worse now is that those who are most unfit for and/or unwilling to be in school are not allowed to drop out, which was not the case many years ago.
Well, the school I was observing had selective competitive admission - and was sucking out the "cream of the crop" schoolchildren from a pretty large area. There were several such schools, all associated with major universities, but such schools were few in number and they were widely distributed geographically. Most other schools [at the "normal" level] were not particularly good, either.
Let's not forget the drastically increased time and effort spent towards self-esteem exercises, PC influenced "history" courses, healthy injections of white guilt, courses on "tolerance", sexual education and various other forms of "navel gazing" since the early 1960s.
Combined with the general dumbing down of critical courses like mathematics, language and science, as well as removing competition between students through grade inflation, affirmative action, and social promotion, it's no wonder we're spending all this money for far worse results.
Don't have kids unless you can educate them yourselves or send them to private school. Also, if your kid does go to a PS, don't think it's any different than any other because they are all de-facto owned and operated by the NEA/AFT cabal.