> agile is a culture as much as it is a project management methodology,

Agile is a culture about project management methodologies, which unfortunately is extremely resistant to actually adopting methodology that reflects it's overt cultural values. Largely, IMO, this is because the agile community has been very bad at operationalizing the “over” statements in the Manifesto into mechanisms by which the right side items are subordinated to the left side items. This results in (for the first statement in the Manifesto) either dysfunctional interaction and demotivated people from constant conflict resulting from the absence of processes and tools, or the adoption processes and tools in a manner which does not respect people and interactions, merely because someone has labelled the tools “Agile”.

The perfect development system will be immune to failure before it's had a chance.

There is no perfect, but one that is fragile to external business realities is probably worse than one that is resilient to it.

(In my company, the problem with agile was that the sales people would insist on always adding some extra tool for a fixed price. We had no choice but to abandon agile.)

> the problem with agile was that the sales people would insist on always adding some extra tool for a fixed price.

What does that have to do with agile? That seems to be a problem that would occur, and be problematic, independent of development methodology.

> We had no choice but to abandon agile.

Seems to me you should have abandoned the offending sales people.

i don’t know what such a resilient methodology might be, because we’re talking about human systems where the participants continuously adapt to emerging conditions.

agile seeks to benefit from the intrinsic motivation of each team member to create something beyond any single member’s capabilities in exchange for empowerment.

when a manager doesn’t understand this principle, that leads to failure (as in your case as well). i don’t know of any systems that can self-correct for this kind of external mis-judgement.

That's the holy grail of management: To create systems that are immune to mismanagement. The Agile Manifesto tried to make clear that such systems don't exist but instead it got made into another system that's supposed to be immune from clueless people.