I agree with you, but this is a controversial opinion which is the root of my frustration. OSI tries really hard to own "open source" as a term and while I'm not against the OSD I am against the gate keeping I see.
Like any person or institution, the OSI is fallible. It's like saying that only a certain organization can define what Red is, and now look what we have with Pantone.
And this is nothing like what Adobe and Pantone did.
The more appropriate analogy would be if the Sugar Free Initiative defined "sugar free" and then you correct me by saying "actually that beverage isn't sugar free, it's artificially sweetened".
The world would be a better, more accessible place if we instead focused on what things are or are not, like preferring "OSI Approved License" over "open source" rather than needlessly conflate complex meaning with otherwise simple terminology.
Sure, let's go with "sugar free". Should I be allowed to decide that it only refers to cane sugar, and so sell food full of HFCS that's labeled as sugar free?
As far as I can tell, it's been a success. The recent batch of nonfree licenses have all had to emphasize that they are not open source in the face of public backlash. Making euphemisms like "source available" unappetizing is exactly the point.
Furthermore, the OSI is just a group, as is the FSF. If the OSI and FSF folded tomorrow the definitions of "open source" and "free software" would not change. It's not necessary for the OSI to approve a license for the software to be open source, so "OSI Approved License" is not sufficient. "Licensed on terms compatible with the Open Source Definition" might be more accurate, but "open source" seems like an adequate shorthand.