"It's quite baseless to dismiss all homeschooling as ineffective," I agree and have not done that.
"...just as it is baseless to dismiss all public schooling as ineffective." That is commonplace on FR unfortunately.
"If you actually believe that most people who are homeschooling 'shouldn't' be doing so (your word), that's astounding." I believe I have said "many" not most. But I know my parents could NOT have given its large size and my mom's working 16-18 hours a day simply washing, cooking, cleaning and running her household and my Dad's working 10-12 hrs a day six and half days a week.
There has never been any doubt that some parents are capable of providing excellent educations I simply do not believe them to be all that numerous.
I do not agree that "many" people who hs should not be doing so. However, I wholeheartedly agree that homeschooling is not right or possible or feasible for every child or every family.
As for your Advanced Physics example, I believe it is you who are weighing apples and oranges. It's not helpful to compare a handful of courses to an entire education---that is, to the educational experience as a whole.
Any child who wants to learn a subject (or whose parents believe study of a particular subject is appropriate) has opportunities and obstacles whether they are in public school or homeschooled. There are children in public schools who don't have access to such courses; they must look elsewhere (online, reciprocal institutions, etc.), just as many homeschool children do.
But to conclude that "many" cannot homeschool because parents, in your scenario, are not equipped to teach some advanced and specialized courses is throwing the baby out with the bathwater. Many homeschoolers start community college in their senior (sometimes even junior) year of high school and have plenty of opportunity to take advanced courses. Under your logic, because a parent cannot teach a child piano, the child should go to public school.
Is it necessary or even appropriate for every child to study Advanced Physics or piano in school?
Even if you are correct that not "many" parents can provide their children excellent educations, that is not even the point. The point is this: can most parents whose circumstances permit them to homeschool provide at least as good an education as their children would be exposed to in public school?
The answer to that question is a resounding "yes."
And if you want to talk "opportunty cost," there again parents whose circumstances permit them to homeschool often can provide their children incredible learning opportunities.