You can’t get promoted at Google by maintaining an existing product. Only building a new one. (I’ve heard that over and over from friends that work/worked at google).
I have seen plenty of people that work on existing products like Chrome, Gmail, etc. get promoted. I also have witnessed people successfully going from L4->L5 as SWEs without launches. I had to see to believe it, and I have seen it. That said, Google is a GIANT company, and other people might have different experiences.
Disclosure: I work at Google.
It's probably just not as fast as being able to hop diagonally instead of vertically