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.