Meh. People post this quote constantly.
I've yet to see a single one of them propose a better viable alternative.
People can imagine all sorts of things. Whether they are viable and superior? That's quite another matter. I've listened to people complain about and express hatred for capitalism and capitalists for decades, but never once have I seen anyone propose a viable, superior alternative, in writing or by any other mechanism. I've seen very few even try.
It's hardly a simple imagining. We also have an ample body of history to support that conjecture. Also, the fact that so many people apparently want an alternative and yet so few of them attempt one (and practically none succeed). Look at how tiny and rare non-capitalist "intentional communities" are, for example.
Completely false. Free market capitalism is the only economic system that tolerates others. If you want to functionally live as a socialist under capitalism, go right ahead. The other way around? Not so much.