I don't get this. 3rd party will never win. Ever. There are two real options. Vote for the one that offers the best outcome for you. Not doing that is accepting that you are ok with the worst of the two, because you had a chance to keep them out of office and choose not to.
How is voting for her the best option. Literally all she can do is benefit the Republicans by pulling votes from the Dems. Hell, in Georgia they've literally ruled that votes for her won't be counted even though she's on the ballot.
Her winning the US Presidential election is less-likely than winning the power all 25 consecutive times by finding the winning ticket on the ground at random truck stops in Malaysia.
context matters. you have no idea what state many people live in. I, for example, live in a reliably blue stronghold. harris will win no matter how I vote. so I don't vote for democrats when when there is a better option on the ballot.
There is no value in voting for a party that doesn't support my values/interests. I personally don't like many of de la cruz's policies, they are poorly constructed, however I think she likely has a better moral compass and backbone than harris does. I'm giving harris until the end of the week to fix her positions on khan and ideally irsael, but i doubt she will so she wont be getting my vote.