a ‘product of its time’.

Something like this, indeed. Or more like a product of the situation, plus a few laws - like network effect (the value a user derives from the OS depends on the number of users using it).

Note that not even the devs are to blame for this; it makes sense someone releasing commercial software would focus on the 70% (Windows), sometimes on the 15% (Mac OS), but almost never on the 4% (Linux).