So here’s a (probably unoriginal) hypotheses. It’s something that’s been floating in my mind for quite a while and I don’t know how certain I am of it. I’d like feedback in the comments if you have any.
In OSR campaigns, if you change the rules pretty drastically mid-campaign, most players would say that they are still playing the same game because to them the “game” means the content of the game that they interact with: items, spells, NPCs, the setting, etc. To this group, the reality and believably of the game world is paramount, rules are just a convenience to make this illusion easier. Altering the rules will certainly alter the feel of the campaign to one extent or another, but nothing essential has been changed.
On the other hand, someone with a storygame mentality (or maybe this is more of a Forge mentality) would would say that those players are now playing a different game, because that tradition sees the “game” as the mechanics by which you interact with the content. To this group, the particular kinds of choices and twists delivered by the rules is paramount, while the content is a servant to that.
I feel like this is the core of the mutual incomprehension I see between the two camps when they talk about each other’s work.