Ensure has been used as a complete replacement for decades. It advertises as a supplement because nutrition is not a one size fits all* kind of thing and being too aggressive about positioning it as a long term food substitute would end in lawsuits.
*Seriously, not one size fits all. Changing elevation will cause you to need more/less potassium for instance.
>Ensure has been used as a complete replacement for decades
No, it has not. An ensure contains a mere 11% of your daily potassium for example. Has anyone ever actually recommended that someone drink 9 ensures a day?
> An ensure contains a mere 11% of your daily potassium for example.
If you are referring to Ensure Complete, (rather than regular Ensure, which is not intended as a complete replacement), it may be low in potassium because complete replacements may be used by people with various medical conditions for which reduced potassium intake, below the usual-stated Adequate Intake, is recommended [1] (its possible for other users to supplement potassium, so that design choice would make it more broadly usable.)
*Seriously, not one size fits all. Changing elevation will cause you to need more/less potassium for instance.