Nathan, nobody is denying that pre-industrial or early industrial relations of production were exploitative of labor. Than means any labor in any country. Claiming that it was only slave labor that was exploited or that it was only the exploitation of slave labor that explains Europe's wealth is absurd on its face, but has unquestioned demagogic appeal.
What is more, exploitation of colonies does not explain the growth of countries that had no colonies, such as Germany. Nor it explains the downfall of countries that did little more than colonial plunder, such as Portugal or Spain.
My view of colonialism as a form of international/intercultural exchange that provided benefits to all parties, but those benefits being conditioned on the internal political and organizational infrastructures of these parties is a better explanation because it can explain more and above all the question that few people ask, namely:
"Why was the progress so slow before the 16th century in every part of the world, including Europe, and so much accelerated after that, again in all parts of the world not just Europe?"
Wojtek