The first time I heard this phrase was in the Dennis Quaid movie, "D.O.A.," one of my favorite films. Quaid plays a college English professor who had two novels published, if I remember correctly. The first one was a blockbuster and got him noticed by the university, the second received lukewarm reviews and the third book just wasn't happening.
"Publish or perish" meant that you could lose your job as a teacher or scholar if you didn't continue to produce work for the establishment who employed you, at least until you reached tenure. Independent authors have the same dilemma, in a way. Stop publishing new books and you will quickly sink into obscurity.
Fellow YouTubers may have it worse than us authors. The video market is most definitely a "what-have-you-got-for-me-right-now"-type of industry. While the occasional cat or puppy video might make several rounds on the Internet, the vast majority have one shot, and a short one. Worse, it can take 8-15 hours of their time to make one 15-minute video, so channels like "The Motorhome Experiment" and "Less Junk, More Journey" are constantly looking for fresh new locations and scenery to film, even if it means driving to the Arctic Ocean, then spend significant time editing, narrating and adding music to their pieces before needing hours of uploading time to get them to YouTube from the middle of nowhere. If they stop for any appreciable amount of time, oblivion happens quickly. Many fans and subscribers who don't get their fix quickly enough simply move on to more developed channels and more content.
I finished my latest manuscript a couple of weeks ago and am working feverishly on polishing and publishing. I will need to spend considerable time on marketing, even with help from my social media team. However, I can't sit on my laurels (do people still wear laurels?) with regards to Book #7. That work has to begin before #6 is even on the market. Even with my relatively modest reader base I was under pressure to give them the new story for their favored characters as quickly as possible.
I became an indie novelist with my eyes wide open. I adequately perceived the market and decided to self-publish, since chances of getting a traditional publishing contract were remote and would take time away from writing. The truth about "publish or perish" weighs heavily on me, but I'm up to the task.