Promotion driven features are definitely not entirely a myth IMO, but on the other hand, anecdotally I saw quite a few people get promoted at Google for doing make-things-work-better work. The trick was figuring out which were the important things to maintain and make small improvements to, rather than just which were the things that seemed fun to tinker with, but weren't as impactful.

"finance tech" -- those people maintaining those systems are middle-aged, comfortable, and well-paid. If you were junior under them, you would never want to stay. Fifteen years ago, there were large fresh hire classes each year. Tons of juniors around the office. Plenty of young ones dreaming up unnecessary "upgrades". Most of that is gone as the industry has matured. If anything, the hoards of junior hires have moved from finance to Big Tech.