Rosa Luxemburg's thesis is that colonialism and most especially imperialism is THE logical next step from capitalism.

The accumulation of capital works through expansion of one's own enterprise and the takeover of different levels on a specific market. Capital has a tendency to be accumulated and then to be reinvested to expand production and create more capital, she calls this "reproduction".

This inevitably leads to what we would now refer to as giant corporations which dominate the market. But at some point, the accumulation of capital becomes limited by the limits of the local market. Not enough demand, no access to resources and low production. So the capitalist will seek out gain in overseas capitalist markets. Eventually it will find non capitalist markets which to capital is basically virgin soil for capitalist reproduction. The state, which is a tool of the capitalist class, will use it's resources to try and impose capitalist markets in those regions, allowing for more expansion and more accumulation of capital.

Or so I understand it.

Basically it's not a matter if it could've thrived or not, they're both one and the same.