You're right that it mustn't be the only prism, yes. But maybe we shouldn't also splinter things like functionality and appearance/usability from the merits of "free as in freedom" either? One of the things that makes FOSS apps work better than alternatives, when they do, is the fact that it's not looking for extra revenue streams all the time with marketing-led nonsense features, bloating the hell out of their product, redesigning just to seem modern (usability be damned), and so on.
And what happens when you have a FOSS alternative with committed and talented devs, a large user base and resources tends to be something truly superior.