The Churchill line is about democracy, but the adapted version is a common variation. It works as a standalone maxim without need of attribution to some famous person.
I don't know if you've noticed, but internet discussions collectively can't seem to avoid "no true Scotsman"-ing what counts as capitalism, likewise its alternatives.
I've seen some people on HN criticise the "socialist" healthcare of the nordic countries on the basis of what Stalin was like, and others saying that China as is today is each of communist and capitalist depending on the point the poster wants to make.
I also clicked through ten pages of Google search results for "capitalism is the worst economic system, except for all the others", each of which showed the literal quotes in the preview excerpts, at which point I became too bored to continue.
I've only heard it said that "democracy is the worst form of Government except for all those other forms that have been tried from time to time", not capitalism and economics: https://winstonchurchill.org/resources/quotes/the-worst-form...