Prioritising the longevity of data can sacrifice the authenticity of what it tries to capture and preserve. When I say authenticity, I refer to how accurate and detailed the data in question preserves a particular state. An original raw image, for example, will capture a landscape much more authentically than written text would. Written text will inevitably contain ambiguity and even bias, if not distortion.
This is why, instead of forcing plain text upon everything, I try capture and preserve data in the formats which store as much information as possible. Plain text is indeed the best format that will likely last the longest time. However, the inability to open a particular file format in the future is, as it stands, a hypothetical scenario. After all, some proprietary binary formats from the 90's somehow can be opened today. It's not that we should go out of our way to use proprietary binary formats, but dismissing or abandoning media files is a much more guaranteed potential loss of information – information which plain text cannot capture due to its limitations.
Indeed, many ideas can be expressed in plain text and don't require more complex formats. We of course shouldn't choose media formats for the sake of using media formats. We should choose accordingly, with simplicity being the biggest priority. Beautifully, the simpler something is, the more guaranteed its longevity will be. Since plain text is practically the most simplest format, I'll of course choose it as the first option when feasible.
But when the limits of plain text get in the way of capturing information, I'll increment to a potentially more complex format. By embracing this approach, I am slowing down both the compromise of longevity and authenticity. Data preservation is a battle between longevity (thus simplicity) and authenticity (thus complexity).
And when we need more complex formats, we should ideally consider open formats – preferably with a standardised specification, and ideally in a non-binary format. Closed and binary formats may potentially hinder longevity; as such, we should seek to minimise our dependence on them.
What ultimately matters is that information is captured and preserved as thoroughly as possible. Between a picture that expresses a thousand words, and plain text file that sacrifices its detail and authenticity, why wouldn't we choose the former? Indeed, this question applies even the choice may sacrifice the longevity. What's the point of longevity, when the pursuit of it can compromise our ability to capture the information we may be afraid of possibly losing?
This isn't an argument against longevity at all. I do strive to ensure my data is preserved and accessible for as long as possible; however, I also strive to capture as much data as possible in the present, even if it requires going beyond using plain text. A simple visual or graphic may preserve the idea more than plain text rambling would. It's why I favour the paradigm of simplicity, rather than the format of plain text. Though it's not my only choice, plain text will still be my first choice!