Long hours are just one variety of unsustainable practice. Accumulation of technical debt, and an absence of concern for individuals' career needs and goals, are likewise acceptable in the context of a short-term emergency but not sustainable.
In perma-Scrum terminology, though, the end of one sprint leads straight into the next. This is the core problem with Agile/Scrum as it's sold: it takes behaviors that are known to be unsustainable and encourages people to carry them permanently, and then says they're not doing them right (No True Scrumsmen) if the arrangement deteriorates. It can't be sold truthfully, as a short-term fix that does a lot of damage if it stays in place for too long-- no one would buy it if it were advertised as something that only works for 6 weeks, max, out of a given year-- so the Agilemongers present it as a permanent arrangement.
I think you are confused. A "sprint" is not "crunch". Its not a period where we work our asses off, late hours, weekends etc. A sprint is just a period of time. For us its two, forty hour weeks. Its sustainable and we've been doing it for 16 months.
It leads to a very confusing situation, where on the one hand the team is lauded for being the most productive team in the company, and on the other, for not working hard enough. o_0
If you can find anything in any Scrum publication that says otherwise then you can make the No True Scotsman claim, but otherwise you are at best misinformed.
