I honestly have no clue how you constructed your conception of "leftists".

Leftists != Tankies != Blue MAGA/Anon != Progressives

Biden did great things while in office. However, the Left has the liberty to continue pushing the current administration to go farther and implement more policies in line with their ideals. This is a force that counters the Right.

Biden did not go far enough on Gaza, and neither did Kamala. The same goes for corporations like those in the Oil & Gas sector that raised their prices following oil demobilization during COVID, as well as those that kept their prices high even years after like those producing consumer goods. Could have gone farther.

When the existing administration makes no concessions to those in the base that voted for that administration but not as their first choice, they have the right to criticize and call out the administration's failure to represent them.

All of this would be fixed, or at least alleviated if we abandoned our political duopoly in favor of an electoral and congressional system that allowed for more diversity in government. Ranked choice seems to be taking off in many states.

Oh, and Kamala was great at the beginning. She took over for Biden, then chose Tim Walz. But then her administration attached itself to Biden, the person that spawned her campaign since the Democratic base utterly rejected him following the debate, and then went after the Right in hope of gaining more votes, taking their own base for granted.

Now that we have the full scope of Kamala's campaign, she ran on much more conservative values than Biden did in his campaign leading up to the 2020 election.

This nuance is not something you'll see in the mainstream media, perhaps not even on Lemmy. But this is the realm that hopefully progressives like me and some Leftists operate in.

Your mindset is honestly the same that many ignorant people share regarding science and the scientific method. Things can be true for different reasons and at different times.