Capitalism can't last forever.Social systems are mortal.Like the human beings who compose them, they are born, mature, decline, and eventually die.The history of capitalist society shows it is no exception. It too has moved from birth through maturity to decline, and is now approaching death.
It might have it's ups and downs but I think it will always rebound eventually. What system could possibly replace capitalism if it did indeed die like you propose?
Something as yet unimagined.
Outright slavery, human sacrifice, or other barbaric practices are unfathomable to civilized people today. The past is usually bizarre from present perspective, as is the future. The future will change us even as we cling desperately to the ways of the past. This is inevitable.
The ways of modern people will seem downright barbaric to our ultra-empathic great grandchildren.