Part of the issue is the balance between the stakes of the current election vs the value of the potential change for future elections. It's possible for someone to be willing to stay home or choose a different candidate as a protest vote during one election, and then view those same strategies as monstrously irresponsible in a different election.

And to add another layer of complexity, keep in mind that both parties are fluid and can change radically over time as factions within them rise and fall.

For example, in some alternate timeline where Clinton got the nomination in 2008, a protest vote against Clinton would have risked a McCain presidency, which would have likely been the most moderate Republican president in modern history. This would have been short term loss for Democrats but likely would have been a long term win for progressives. The Democrats would likely have shifted to the left as they sought more candidates that appeal to their base, and the Republicans would have had their more moderate wing exerting greater influence and filling their leadership positions.

The situation we have today involves very high stakes, in that Trump and pals are threatening serious damage to the basic principles of democracy and rule of law, in addition to all of their horrifying policies. And the message that the Republican party will get from the next election is especially critical. Trump won in 2016, but they performed poorly in 2018, 2020, and 2022. Their shift to the right and the purging of anyone not 100% loyal to Trump has lead to a significant brain drain and a shrinking of the party.

A Trump victory would help the worst people keep a stranglehold on the party, while another defeat would send the message that their current path is a dead end. There's a sizable portion of the Republican party that isn't particularly happy with the MAGA crowd, but who are willing to go along with them if it means winning, and others who are just trying to keep their heads down because dissent is punished harshly. The power struggle that would occur after another Trump loss would very likely push the party to move back towards something resembling sanity and competence.

Hell, just being rid of the 800 pound orangutan in the room would make it easier for both sides to work together on the things that shouldn't be partisan. We didn't have a problem getting Ukraine aid passed until Trump started exerting pressure, which only got worse when he vetoed a speaker candidate that supported Ukraine aid in favor of the current one who is more than willing to open his ass cheeks for Trump's puppeteering hand.