There's a difference between entailment and implicature. Implicatures can be cancelled using a negation. Entailments cannot.
"Fido is a brown dog" entails that Fido is brown, since the following is a contradiction: "Fido is a brown dog, but is not brown."
"John walked to the cliff and jumped" merely implies that John first walked to the cliff and then jumped, but this implicature can be cancelled, as is evinced by the acceptability of "John walked to the cliff and jumped, but not in that order".
Sentences with "until" might imply that an event will occur, but they can certainly be cancelled, as evinced by the acceptability of your sentence "This machine will keep working until the button is pressed, which will not happen."
In short, "until" does not entail that the action will happen, but it might imply it.