Programmers can also create non-enshittified solutions in their off time and release it publicly for free and many do.
It's a nice idea, but what that ultimately boils down to, is horrible to use programs with barely any support because people (reasonably) don't have the time or support to give them the full beans.
They also result in programs that are basically only made for their users, and that, to everyone else are some weird esoteric programs with outlandish UX.
Like I like open source as much as the next guy, but open source software is not fixing the enshitification of society, especially as devices take away more and more user autonomy. These are bandaid solutions that allow the power users most equipped to make arguments against the hostile enshitification takeover to bury their heads in the sand as they scurry to stay alive, squeezing between the cracks, increasingly having to give up in more and more areas as solutions take more and more focus to keep alive.
Also, more than that, how many programmers actually are doing this? I'd say it's a rare occurrence.
What good can a marketer do for the world?
Activism. They communicate and convince people, so activism is an area where they could help the world if they so chose.