Liberal/welfare democracies didn't stop growing because there was no room to grow.

Gotta partially disagree with you on this one. After WWII, imperialists got a free pass to expand throughout most of the world. All the way up through the fall of the USSR, the US was privatizing and gutting the public sectors of their client states and the eastern block.

However, with the neoliberal consensus overtaking most of the world, the US didn't have any more big markets to crack open, which is why they had to turn their sights on gutting their own public sectors which they had free access to. Foucault's boomerang and such