I'd suggest that the protests against Trump are still in that early stage. The Civil Rights Act didn't spring fully formed from MLK's head after the march on Washington. It took years of work to get to that point. Were all of the previous protests therefore useless? I don't think so.

Exactly. That's my point. With "No Kings," Democrats are trying to skip straight to the end (the biggest possible rally!) without doing any of the things first that make the biggest possible rally an effective culmination of a strategically sound protest movement. That make it clear what the big rally is actually for (or against), rather than just being as many people as possible with as many agendas as exist in the Democratic Party.

The previous protests were not useless - they were critical. And they were focused, narrow, and coherent.

Again, nothing wrong with having a big old rally to protest everything. It's just not likely to have any different impact than all the Resistance rallies in Trump's first administration, which the OP recognizes didn't really amount to much.