And the third paragraph kind of lays bare the problem with sprints: The PM can only look at all the work that needs to be done if, well, there was a discovery phase to list all the work that needs to be done. That discovery looks suspiciously like planning.
That's why agile focuses on continuous delivery of value. It's excellent for projects where it's hard to estimate the full scope, but there are many intermediate targets that are valuable to the customer. You go along until enough value is delivered, and then move on. Not all projects work like that. Some things only generate any value at the end of a long slog. Some things can be delivered incrementally but require a predictable end date. (Hello, Black Friday!)
And so, you get to adopt methodology to the problem at hand. Sometimes, you really need to spend the planning up front. Sometimes it's enough to sketch it out on a napkin. Sometimes you can do things sprint-by-sprint. Sometimes, you have larger checkpoints and sprints in between. The idea that any single methodology works for every problem is an idea favored by the consulting industry, but not a reality. And it predates agile, by a lot - I've been a process consulting victim way back in the early 80s. (And the mass of miracle methodologies directly led to "No Silver Bullet")