I generally agree with the points that user27280 has raised, but I wanted to give some additional context here.
From the viewpoint of modern English, "must-be" sounds outdated and perhaps even hard to understand for some native speakers. So I was to translate the passage in question I might not use such a phrase.
However, 当たり前品質 looks like a domain-specific term which may have been invented by Kano (狩野), and seems to be translated frequently as "must-be". (See the English Wikipedia page on the Kano model)
In light of this, while a bit cryptic and aged, "must-be" does seem like an apt translation given the fact that it is a single (compound) word that expresses the concept efficiently. As user27280 mentioned, it would be possible to translate as "considered as obvious", but that would be too wordy and clash with the feel of the original term in Japanese. Note, it could be argued that "当たり前な品質" would be a more everday way of expressing this in Japanese.
Furthermore, even if someone wanted to translate this term in a different way and use a more modern-sounding English expression, it might be frowned upon since it would deviate from what appears to be an industry standard term.