So I understand why the Machines gave Neo the opportunity to defeat Smith. Smith was uncontrollable and the Matrix was practically lost. Neo defeating Smith was (perhaps or perhaps not) a long-shot from the perspective of the Machines, but the Machines were backed into a corner so-to-speak.
However, once Smith is defeated and the Matrix is reset, what incentive do the Machines really have for upholding the peace deal? Why couldn't the Machines have simply lied to Neo, used him for defeating Smith, and have proceeded with purging Zion as they did all previous cycles?