Every success we're likely to see in our lifetimes is going to be a qualified one, just as every failure will be a qualified one. It's more of "these groups of outcomes are worth celebrating/praising and these other groups of outcomes are worth lamenting/damning" than a crystal-clear definition in either direction.

Three examples:

1. Say there's a U.S. president who pulls us completely out of Iraq and Afghanistan, who ends our military actions that directly kill people (e.g., our perpetual drone assassination program), who stops supporting actions that directly kill people (e.g., refueling Saudi bombers over Yemen), and who shuts down 30% of our foreign bases. There's a lot of "success" in this scenario -- we're killing fewer people, we're taking meaningful steps towards ending U.S. empire, etc. -- but there would still be tons of work to be done to achieve socialism.
2. Say we get M4A but nothing else changes. Again, lots of "success" here, and again, tons of work yet to be done.
3. Say we achieve something that could fairly be called socialism. The American government is explicitly a socialist one, we've undone empire in a way that makes that first scenario look amateurish, major industries are nationalized, smaller industries are entirely filled with worker-owned enterprises, all the basics of decent living are fully guaranteed, etc. Absolutely a "success," but we'll still have "failings" when problems occur (and especially when we handle them badly), and we'll still have "failings" where we insufficiently advance the conditions of people this country has historically ground underfoot.