The problem is that people think "maintenance" is a bad word. A company like Google is seeing growth from external customers, churn from their internal systems as internal best practices change, and changing threat landscapes on the internet.

"Maintenance" often is actual hard feature work. But when your org or company has a culture of thinking of maintenance as some low-level job, you get a culture like Google's.

I don't disagree. I don't think maintenance is a bad word. But I do think that there is a point beyond which "maintenance" cannot grow. If you're maintaining a system there is a breadth of experience you cannot get. Being a senior engineer at Google is not easy. But my point is that sre is still fully capable of supporting L7 and L8 ic swes, while "maintenance" isn't.