The process is supposed to be sustainable. That doesn’t mean you can take one activity and do it to the exclusion of all others and have that be sustainable.

Edit:

Also, regretably, I’m using the now-common framing where “agile” === Scrum.

If we wanna get pure about it, the manifesto doesn’t say anything about sprints. (And also, you don’t do agile… you do a process which is agile. It’s a set of criteria to measure a process against, not a process itself.)

And reasonable people can definitely assert that Scrum does not meet all the criteria in the agile manifesto — at least, as Scrum is usually practiced.