Typefaces of this quality and scope come along all the time. I don't really understand how it's a big deal. Take Source {Sans, Serif, Code) Pro, by Paul D. Hunt

Typefaces of this quality and scope come along all the time. I don't really understand how it's a big deal. Take Source {Sans, Serif, Code) Pro, by Paul D. Hunt for Adobe (also open source); it's arguably a better typeface for user interfaces, and is a much better interpretation of Franklin Gothic and related Morris Fuller Benton gothics (as Plex claims in some areas.) Also Apple's San Francisco, admittedly missing a serif variant, is a similarly broadly scoped project... of much higher quality. Both of the above typefaces have fonts with character sets / glyph counts exceeding Plex: Plex Sans Regular - 663, Source Sans Pro Regular - 832, SF Pro Text Regular - 2439. IBM Plex as an open source font is cool, however I think is of limited use to the broader design community... the formal qualities carried over from Paul Rand's IBM logo make it too idiosyncratic to use outside IBM products. For IBM, I think it's awesome, especially considering it's replacing Helvetica. Also, your terminology for figure styles is inaccurate. Proportional means of variable horizontal width (not monospaced), where the alternative is tabular figures (monospaced for tables). Old style figures can be either of the above, however their form is more like lowercase characters (an 'old style') that tend to work better in continuous text.