Its not well-founded because modern financial instruments such as retirement savings blur the line between those who labor and those who command capital.
Not so much once you don't put the distinction at "owns at least one share and owns nothing". If you plot the amount of ownership, there are very clear clusters.
By your reading, no theory can ever be known to be well founded, as there is always the possibility of another discovery finding phenomena that aren't explained by the theory. I believe that most people would view science (in the wide sense, not limited only to disciplines that apply the scientific method) as a continuous process of abstraction and refinement. Newtonian mechanics is pretty accurate if you limit yourself to objects of a certain size and a certain speed, a limitation which is reasonable considering those are the sides and speeds we encounter most often in our senses.
That may blur the line between proletarian and bourgeois, but it does not invalidate the idea of class conflict within Marxism nor Marx's other critiques of capitalism such as commodity fetishism and alienation. Although you are very correct that Marx didn't anticipate modern financial phenomena, there has been active work within Marxian economics for the past 30 years about this, most importantly Anwar Shaikh takes it upon himself to do a semi-Marxist analysis of it in Capitalism, Conflict and Crises as does Kliman.
