Marx also saw the potential of sustained economic growth way before anyone else did. He thought that socialism itself would only start making sense and become actionable after capitalism had exhausted its own growth potential; this is what would have made socialist revolution a foregone conclusion, a necessity of history - and an intermediate step towards full post-scarcity, which is what he meant by 'communism'. If he deserves to be called an economist (and I believe he does!) it's for these things. Not really for anything he wrote in Das Kapital.