Let us first agree on what we are arguing. You proposed that:
> For most software projects, there should be a way to buy a commercial support license.
> For many, such a license should be required for use in any org which is over X employees.
I interpret this to mean that: You impose in the license of the software that if you are an organization over X employees, you are not entitled to use and distribute the software under that license instead you must hold the commercial license or you must in addition to the normal license hold the commercial support license.
In the first case(where the normal license grant does not apply to sufficiently sized orgs) I believe you impose a restriction of use on the software in a specific field of endeavor in contradiction with section 6. in the OSD.
In my view it is clear that if you require an additional commercial support license for some users but not everybody you violate section 7. as the rights to the program is restricted for some parties (orgs with over X employees only has those rights if they also acquire a commercial support license):
“7. Distribution of License
The rights attached to the program must apply to all to whom the program is redistributed without the need for execution of an additional license by those parties.“
Now it might very well be that you had another mechanism in mind for imposing your requirement of “Orgs over X employees must hold a commercial support license”. In that case it might very well be a different mechanism in the definition that conflicts with your mechanism, but I cannot see how you can achieve your aim of imposing a requirement on usage of the software for some users without contradicting the spirit of the open source definition which says that you must grant the same rights to all users of your software.
I cannot find in your link [2] what you mean by the phrase: “In practice, there are many "Open Source" licenses that allow paid licensing [2]”. Usually when people speak of paid support licensing for open source (or free software) that is an OPTIONAL support license or a separate commercial license(see the QT license options) that you acquire the software under, and it does not conflict with either free software or open source software. My opposition is only your stipulation that the commercial license be required for some users of the software, and it is this requirement part that I claim conflicts with the Open Source Definition. Note that it is perfectly valid to charge people to obtain your open source software from you, but you are not allowed to impose restrictions on their ability to redistribute that software (including source) to others for free, hence this is probably not the solution you are looking for?