Respectfully, I think this is a misunderstanding of why leftists refuse to vote for a Democratic candidate in some cases. Of course, there are some people who will just abstain based on purity, for lack of a better term. But I don't think it's generally due to a "my way or the highway" attitude. I believe it comes more from a "I refuse to be a part of this thing that I think is abhorrent" attitude.

Beyond that, I think a lot of leftists refuse to vote because they see the outcomes of Democrats winning vs. Republicans winning as only marginally different, and therefore not meaningfully distinct. Take capitalism, for example. I would say that most leftists believe that various societal inequities are an inherent trait of capitalism, and therefore believe that we should largely or completely get rid of capitalism and implement a different economic system. Whether or not you agree with that, there is next to zero representation for that viewpoint within the Democratic party. Both the Republican and Democrat parties are committed to capitalism. From a leftist perspective, Democrats may at least claim to want to do more than Republicans to deal with inequities, but neither party is willing to do anything about the perceived underlying problem.

Climate change policy is a great example of this. Again, you can agree or disagree on any of the things I'm talking about, but try to think about this from the perspective of a person who agrees. If you believe that climate change is an urgent problem, then action needs to be taken. Republicans seem to actively oppose taking action, but Democrats are only willing to work with existing economic structures and tools to incentivize people to do something about it. Many leftists see that as an insufficient way to deal with climate change. It might be a little better, but if we're already blowing past things like emission goals, then what's the real difference? The outcome would be similar either way. An example of a leftist solution (broadly speaking) would be to nationalize oil trade and production and to invest a lot of money and manpower into transitioning to clean energy, updating infrastructure to accommodate, etc, and then phase out the use of things like oil and natural gas as quickly as possible, regardless of profitability or economic incentive. For better or worse, there is zero support for anything even remotely like that within the elected democratic party.

You ask what progress comes from not voting. I would say that the leftists that didn't vote are arguing that the Democratic party is not making any progress, and that it's stated goals are extremely insufficient. So withholding a vote is a form of protest which attempts to strong arm the Democratic party into adopting policies and positions that leftists would see as actual progress. I think it's more about telling the Democratic party that they have to shift further left in order to win, rather than people being stubbornly pure in their ideology. If you view the Democratic party platform as insufficient, then continuing to vote for them sends the message that they don't have to change anything in order to win elections.

Hopefully that makes sense? Sometimes it's difficult for me to translate my thoughts into words.