But the term "sustainable pace" is also mentioned a lot in the literature.
An increase in velocity does NOT mean "work harder"; an increase in velocity should come natural as your codebase expands, you get more reusable components, your team gains experience, etc.
Example. New feature, estimated at 5 story points. One part is a button, takes you half a day to build it.
Next feature, very similar to the other one, 5 story points. But this time you have a button component already, so you can just reuse it. Same points delivered, less time spent, meaning you have time to pick up another story, meaning your velocity goes up.
Nobody, least of all proponents of scrum / agile methodologies, is saying that you should work harder, do a death march, etc. It acknowledges that software development is an infinite and long term process.
Scrum does NOT work with deadlines. It works with predictability; your team's average velocity and the story points of upcoming stories are known, it's then up to the product owner to set priorities if they need a feature live at a certain point in time. Missed deadlines then become bad planning and prioritization more than employees not working fast enough.
> There is not rational purpose to this.
Just to highlight this: The rational purpose to sprints is for project management to be able to look ahead and be able to say "we expect this feature to be done in X weeks time". It doesn't mean that a 4 day task should be done in 2 weeks. A sprint is just a unit of time, it isn't a promise.