Ironically enough "this is our way , take it or leave it" would not work for Pharo because its smalltalk and basically smalltalk by architecture allow you to deeply modify the system from the get go.
This make Pharo technically impossible to control from a dictator and committee point of view like lets say Python or Linux. CPython is a single implementation , but with pharo every pharo app is essentially a new pharo implementation.�� The moment you modify or extend the pharo image you make a new pharo implementation.
I don't like the tone Stef is expressing , he is quite rude and definitely does not represent the tone of the community which far more open to dialogue but he is correct , GPL would never have worked for Pharo. Actually I dont think I have seen a language that is fairly popular under a GPL license.
There is of course software under GPL which is sucessful commercially, Blender is an example, but GPL does not cover 3d assets, music and sound. In that case you use another kind of license like creative commons or heavily modify GPL to extend beyond code. So it was definitely not GPL that made Blender popular, actually it caused a problem with game developers because games using the BGE (Blender Game Engine) were at first considered data because the code was packaged inside the blend file which had a binary format so that meant it was not covered by GPL because it considered the whole game code just data (there is a separate executable for loading the game code)�� but then Blender decided to change this also to GPL with extending its license and that pretty much killed commercial games made with Blender.
So technically you could get away with GPLing Pharo because you could argue that Pharo image is merely data that the VM loads and not real source code, which is kinda correct but it would be messy and the legal interpretation very confusing and uncertain ( leaves a lot of room for legal interpretation ) . As a company you cannot risk this , especially while you expect to make big profit.
As stef said GPL is like a virus, it spread anywhere it touches. Even if all you do is add a tiny bit of GPL code inside the Pharo image would turn the entire Pharo implementation including the VM into GPL and because Pharo tries to approach as many companies as possible as most other languages do , because money helps improve the popularity and the quality of the code, MIT is definitely the way to do.