Didn't it cut poverty by like one third? I don't see how that can be a failure. Would have been an even greater success had Nixon and Reagan not gutted it.
It slowed the rate of decline in poverty. Cost growth in Medicare/Medicaid was effectively 100% of the public debt before the 2008 credit crisis and recession. Medicare and FICA tax increases have caused a long-term increase in under-24 poverty rates. An encyclopedia could be written about the failures of the Great Society and the New Deal.
The Great Society/New Deal have only one accomplishment -- it made people believe that Democrats cared about people and that Democrats had the moral high ground.
Unfortunately, collectivist macro policy is an invention. It needs constant updating and adjustment to keep with the times. American collectivist policy was poorly conceived from the outset, and a majority of the people are dedicated to keeping it as primitive as possible.