I'm usually skeptical of protest votes, but these primary protest votes are actually effective for the same reason voting is effective. Politicians aren't as static as we want to believe- their number one priority is reelection. When you vote, you are telling the candidates that you are politically useful and they will begin to pay attention to your needs. When you vote for all levels of government, you are giving deeper detail into what your needs are.

If a significant portion of a district votes for a Democrat as president, and then a Republican for state representative, the democratic administration will likely make more conservative decisions if they see that portion of the electorate as critical to reelection.

This naturally reveals a big problem with the electoral college, as there are maybe 5 states with critical demographics needed to win reelection. The people in Ohio, Nevada, Arizona, Georgia, and Michigan are more influential to the type of administration a Republican or Democrat would run.

If the Gaza protest vote happened in Washington State or California, I doubt Biden would have reacted as much as he did for Michigan. That said, this problem politicians face goes from the President of the US down to the president of your HOA. So I'll acknowledge that the power of voting fluctuates depending on the specific outcome you are measuring, but there are so many variables. In political science you need to learn to live with paradoxes, and this may be one.

Either way, I believe we should keep voting; they are paying attention.

There is a whole calendar

https://ballotpedia.org/Changes_to_the_2024_Democratic_presidential_primary_calendar

There is still a primary, but the incumbent has only lost his party's nomination 4 times, only Democrat Franklin Pierce in 1856 was elected the other 3 ascended from Vice President. It has never happened since the modern primary system was created in the 1970s.