On the first bold word, I'm not "assuming." I'm "proposing." My proposition is "what if there is design tightness and resistance to (foundational - you omitted this) modification in a TTRPG system (just as there is in biological systems, plenty of engineered systems and in plenty of art). You've inverted what is happening here. You are assuming that there is no such thing as a level of design tightness and resistance to modification. You then index this assumption (that, given our interactions over the years, is clearly the cornerstone of your ideas around TTRPGs) when you relate your ideas on "the essential philosophy/culture of TTRPGs (or something like it)" directly thereafter (the second bolded part after "assuming").
Your position (which I already knew) is "there is no such thing as system's say (or designer's say) because GM."
I already knew that about you. I'm curious why that is.
Why that is is because, in my view, part of the GM's role is to set the parameters and rules of the game...though of course this can also be done by mutual discussion and agreement among all; which means the players come into it as well, muddying the waters even further as to who "says".
Note I'm talking here largely about what happens before play even begins, when the GM and-or group is deciding on a system and-or if-how to modify it.
So this is actually useful to our conversation. This is what I was trying to tease out.
So, beyond the TTRPG essentialism/"beating heart of TTRPGing" you're espousing above (and have espoused many times before), you're actually talking about how the sausage is made here. So, presumably, the driver's experience, music consumption, and art consumption might/would all change, at the macro-cultural level (just as you've specified your idea of a macro-cultural essentialism of TTRPGs), if humans felt (I use the term "felt" here very deliberately) that they had the "the chops" & means (skills, tools, kit). All cars, music, and art would change status from "completed works" to something like "pending DIY projects?"
Pretty much, yes; people could put their own stamp on things and see if they could do them better. And some in fact would make improvements, while many would not.
Obviously things like copyright and patent laws etc. make much of this merely hypothetical. But take, for example, a band doing a cover of someone else's song. The new version could be better, worse, or note-for-note the same as the original, but it's not the original: that cover band has taken that song and put their own stamp on it, even if they didn't create/write the song themselves.
If I had the (probably ridiculous amount of) money it'd need, I'd love to be able to take an ordinary car and get it modified such that it more closely does what I want it to do in the manner I want it done.
Do you think that might occur? If you don't think that might occur, maybe break out why TTRPGs are particularly unique here in the way that cars & driving experience and music and art consumption are not (indexing the owner/primary participant relationship with them if you would...because that most closely maps to something like your personal usage of "GM").
Why TTRPGs are unique here is that - in comparison with art, music, cars, etc. - many (most?) people
do have the chops and available means to modify them and make them more bespoke to their own wishes/needs/desires.
And further, where even in their infancy most games - and most products in general - are presented and marketed (intentionally or otherwise) as "here it is, take it or leave it", and thus that's what we as consumers have become accustomed to; RPGs - particularly D&D - were initially presented as more "here's a mostly-complete framework, make what you will of it"*. They expected and in some cases required modifications and additions in order to become a) bespoke to the table and b) in some cases, playable; and that process would almost universally be done by the GM either before play began or ad-hoc during play as issues arose. (remember, in theory the DMG was off-limits to players)
And so, the philosophy of RPG rules - in rather stark contrast to the rules of most other games - being seen as malleable to suit each table and-or GM came about; helped immensely by the fact that all it took to do so was a pencil, some paper, and some thought and time. Thus, in some ways the GM and the system became one and the same.
* - Gygax in his 1e DMG was famously conflicted on this, arguing for uniformity in some places then exhorting DMs to kitbash the system in others; looking at it now I suspect this conflict arose from an inability to square his own tendencies as a designer/kitbasher with his/TSR's desire for uniformity in order to sell more books.
Can you engage with those questions/propositions above?
Is the above what you seek? If not, please clarify.
* In case its not clear why I used "felt" above, I would hope its abundantly clear that the human expert class is overrun with a hubris that renders their perception of their own expertise significantly more a burden than a boon (even in the discipline that they're supposed to be an expert in). Ironically (given that you believe GMs DIYing is the backbone, or near enough, of TTRPGing), I would say GMs in the TTRPG community are like the paragon of this phenomenon.
Not sure if you're saying that "experts don't feel that they are experts" or that "experts think they're more expert than they are". In either case it doesn't much matter; the key point is the ability to be - and feel - able to do these things rather than the quality of the result as seen by anyone other than the doer.