To make things short, Marx never observed imperialism being used to bribe the proletariat out of revolution in developed countries. Imperialism is what caused revolution outside of the global north (generally). Imperialism, like capitalism, is a decaying system, though, and has merely bought the bourgeois imperialists more time before revolution. That's why we will see revolution in the global north, imperialism is in its death throes and the working class within the global north is becoming more radicalized as the bribes run dry.