Yes, that's the point, that after each week (or two) of work, we've learned more and the plan already is outdated and needs to change.
> inputs likely aren't changing with that frequency
In my experience, they absolutely are unless you're building the simplest CRUD app that's identical to one you've built before. The inputs are less often changes from external stakeholders, and more often tasks that are turning out to be more difficult than hoped.
> and you aren't going to be able to pull in all the other stakeholders that often anyway
Of course you are. It's the PM's job to gather the new inputs, go check in with relevant stakeholders (takes a day or two at most), update or re-confirm their priorities, and then make the updated decisions for the next sprint.
In my experience, 15% of time spent each week on planning is about right. It doesn't maximize the productivity of each person coding, but it hugely maximizes the productivity of the whole team in delivering a valuable end result.