Spoilers? You mean "third party" candidates labeling themselves as Democratic Socialists or something?
They have a proven draw of something like .05% of the vote. Democrats can weather that storm. More likely there will be more Millenial Progressives elected to the house in 2020, some of whom will be Democratic Socialists rather than Democrats. Democrats have a much better chance of taking the Senate in 2020 too and you can bet some genuine progressives are going to be running for the available slots.
If Democrats can't field a candidate able to handle third party votes and easily defeat a weakened and unpopular Trump then they simply don't deserve to win the presidency.
"Be yourself; everyone else is already taken."— Oscar Wilde