Let's say your correct and the founders didn't know ebay could scale (even though this was during dotcom bubble and raised millions of dollars in investment) so what?

That doesn't mean it's not possible or that a savvy entrepreneur shouldn't estimate market size today.

The flying car example you give is correct. Yes, I fully agree that a flying car (at right price point and specs) would have a huge TAM. That doesn't make it inherently investible (though I nite a number of companies have received investment in this space). There is a huge difference between a huge TAM is desirable, maybe even necessary and a huge TAM is all you need. You seem to think people are arguing for the latter, when they are arguing the former.

I fully agree that the educational software space maybe huge. As such building an app to capture a portion of that might well be worth doing! I think you just demonstrated the power of thinking in terms of TAM - while X doesn't look huge on the surface, it's actually just the first steps to Y which is.