No human system is going to be perfect, ever, because people have free will and can use that will to do good or bad.   But saying Capitalism can't last long in the real world is like saying, a crime free society can't last long in the real world.  That doesn't mean anything.  We should still always push for that goal, push for a society where allocated by choice, and not under alternatives that usually involve deadly force.  Instead of human will being treated like a threat, we need to take advantage of it exactly like capitalism does.

Also, lets be careful about what we call Capitalism.  For example, what part of free market Capitalism would have the state take the dollar off gold by presidential edict and impose a central bank that effectively prints up money for the elite to go into housing, stock bubbles, and govt spending while the rest of us get saddled with inflation, taxes, and debt?  Not to mention all the zoning laws that enhance these housing bubbles, and screw over the homeless, not to mention all the regulations that make health care up 10x more expensive than needed.   Yeah, capitalism my ass.   The USA is a socialist shit-hole.