When I interviewed at Google a decade ago I asked the interviewer that directly and he admitted most of the work will be maintenance & that sort of challenge (he was a former CS professor).

This was for natural language processing at search so very much the foundation of google's bread & butter/cash cow (pick your favorite metaphor) and something you may think requires pretty sophisticated work. But I guess by then a lot of the fancy work has already been done & I'd have been one of 1000s of programmers working on it.