Is Sanders the socialist Goldwater?

[Cassandra]

Obviously Sanders failed to get through the primaries. Nonetheless, his primary campaign was a energetic, activist-driven movement and his ideology seems to have quite a lot of purchase, especially among young democrats. Will he be seen by history as a Goldwater-esque figure, someone who lost so that a future generation of radical leaders could take over the party and eventually win the presidency?

[TiltsAreUnderrated]

Sanders has done a lot to accelerate progressive change so would meet the criteria you lay out, but I don't think those are all that's needed for a Goldwater-type figure. Goldwater's fans felt cheated by a verdict delivered from the general electorate and rallied the Republican base against that in future contests; Sanders didn't get the nomination, so can't do that (and thus the question of whether a landslide loss in 2020 would cause the Democrats to double down won't be answered).

I also think his most obvious "successors" as the frontrunners in the progressive lane (insofar as a lane exists) have worse chances in presidential primaries, ceteris paribus.* I have serious doubts about AOC's ability to retain large parts of his coalition, let alone reach out to those lost to Biden. I feel similarly about Warren, although a bit better; her 2020 run would probably boost her in the next contested primaries, but it's also shone a light on poor judgement that hasn't completely disappeared since.

Baldwin and Merkley might be better positioned, but I know a lot less about them and they seem less likely to launch bids.

*The worsening of material conditions/unpopular incumbent Democratic administrations/a string of defeat for moderates/the base becoming more progressive/worse "moderate" candidates may well override this factor, but I don't think they're particularly strong national candidates at the moment.