Who is the we you are referring to? There are plenty of us who a pretty cognisant of a range of RPGs, a range of approaches, a range of techniques, etc.
No single person in particular. There is a rather popular strain of thought right now, that can be summarized
very loosely as, "D&D can and should embrace the
entirety of these ranges." That one system not only can, but
should, simultaneously serve the needs of neo-Gygaxian murder-hole delvers, "systems are toolkits" (or "we've got a binder of house-rules") hobbyist re-builders, hardcore "set the system running and find out what happens" simulationists, casual beer-and-pretzels dice-slingers, folks who love deep and balanced systems, etc. without any of them feeling left out or under-served.
But then again, I'm of the mind that a return to a two-system setup would be more beneficial than detrimental. Both using the same fundamental chassis, so there's interoperability and relatively easy translation between the two. One specifically geared for low mechanical engagement, extremely straightforward rules, maximal ease of inventing your own rules on the spot, a casual attitude toward any "official" patching of holes in the rules, etc. The other specifically geared for high mechanical engagement, strategic and tactical depth, a strong emphasis on innate balance, built-in support for "simulationism" (which, from multiple fans of the term, I find usually means "system maps pretty close to world" plus "player inputs lead to surprising, yet deterministic, results")
and for narrative-driven play. I would
not call them "Basic" and "Advanced" because those are super loaded terms, but the fundamental idea is similar. TBH I really don't know what I'd call the "modern day AD&D" because calling it "Modern D&D" is likely to cause confusion due to "Modern" being a term for
settings or
rules options where stuff like guns and telephones are in the rules, but I'm at a loss for a better alternative word ("Contemporary" would be a candidate...but it starts with the same letter as "Classic," unfortunately).
If you want to call that a ruleset, sure. Go for it.
I mean, how is that
not a rule? (And therefore, trivially, a set made up of one rule.) It's literally the foundation of monarchy: "Do what the monarch says." Likewise for dictatorship and autocracy. And, historically speaking, those things remain
the most commonly-used system (yes, it IS a system!) of humans organizing themselves. "Do whatever [X person] tells you to do" is, most certainly, the rock bottom most simplistic, primitive "rule" you can have, cutting things down so far that, if you removed literally anything else, it wouldn't be a rule anymore. But it does seem to fulfill the requirements: "a principle or regulation governing conduct, action, procedure, arrangement, etc." It governs conduct, because it tells you what to do. And it is a principle, albeit one so simple it only barely avoids being vacuous.
To phrase this differently: Rules can also be understood as defining what you
aren't permitted to do. In a situation where you have no system
whatsoever, all behaviors are permitted, full stop. But, in the situation described, that's not the case. There are behaviors that are not permitted: precisely those behaviors that the DM disapproves of, whatever that may end up being. Is that unreliable and difficult to predict? Sure, because humans are both of those things. But a rule isn't required to be perfectly reliable and predictable; if it were, we couldn't use dice (or any other random resolution method).
There is no competition for most games. The PCs may win the day or they may go down in ignominious defeat. They may be heroes that stop the apocalypse of the week or build up the local thieves guild. How is anyone going to come up with what it means as successful play with all the possible options other than ... wait for it ... having fun.
But isn't that exactly the problem? You can't design a game with
only the goal of "have fun." Because, for
some group in
some context, literally ANY activity you can define can be "fun." To design a game "for fun" is like saying you design a dish for "tastiness." Some people find durian tasty; I think it tastes literally the way garbage smells on a hot summer day. I find cilantro delicious; others think it tastes like soap.
Every ingredient that won't outright kill the person who eats it
can be tasty to
someone, somewhere. This particular design rule is useless for actually limiting behavior relative to the design of games (or food, or anything else) because it is identical to saying, "Just do
good things and not
bad things, 4head!"
The rule "design a game so that it is fun" only becomes useful when you start picking
specific people (or groups of people),
specific situations,
specific methods/tools/styles/ideas. Once you have that, you can start narrowing down. Just like, for example, if you narrow "food" down to "Italian-inspired pasta dish," the rule "design a dish that is tasty" actually becomes a useful guide, because you have a context within which to work. Devoid of that context, there is no value to "design a [X] so that it is [good]."
This is exactly where the assertions that seem to trouble you so much come into play: they are not about "design a [X] so that it is [good,]" but rather about
choosing your audience and situation. It's also where my above comments are centered. I think it does a disservice to the TTRPG community that D&D is trying to be everything for everyone (or, rather, that it
pretends to be trying to be everything for everyone, while still actually being only for a selective subset.)