Capitalism only exists for like 500 years. In some places of the world it did not fully develop until the 20th century. It is very young.

Are you confusing capitalism with class society in general? Yes, class society exists since the neolithic revolution. Those economic systems had barely anything in common with capitalism though. Even medieval feudalism is very different.

And yes, late stage capitalism is a term. It was coined by Lenin in his book about Imperialism. You might not agree with the term but that doesn't mean it is not real.

I think it is very obvious that 20th and 21st century monopoly capitalism is qualitatively different to 18th and 19th century free market capitalism.