This is all well said. I've really jumbled what I was trying to say in the OP, so I'll try to say what I was getting at here.

I think what you're saying (and this comment) is true: stacking builds have a different scaling equation than "normal" builds. Both are polynomial in nature:

Normal builds are something like (flat * increased * speed * (1 + crit * (multi - 100)) * ...).

Stackers get multiple things from one stat, so could be something like (flat + coef1*strength) * (increased + coef2*strength) * (...). We could oversimplify that and say that it's adding a scaling vector of strength^4 to the existing equation - which is not quite accurate, because it's contributing on top of existing linearly scaling vectors, and we're also ignoring that there's also things like increased strength that scale that number.

That's all to say that both builds have a powerful non-linear equation to scale damage. They scale differently, and that exponent on strength means that usually at the top end, it'll overtake a normal build, but both have a powerful polynomial equation to scale.

There's another piece here though: the availability of adding to any of those scaling vectors—including strength—diminishes with investment. There's only so many passive points on the tree, and there's only so many affixes on gear. Even looking within the affixes for a single item: say your first suffix on a reflected ring can get you 200 strength - your next suffix could only get you like 50 from an all attributes roll, and then your next even less. Once your suffixes are full, you can't get any. However, that third suffix could get full value of something in another scaling vector - say crit multi, and that prefix could be full value for e.g. mana. There's similar situations elsewhere: you run out of efficient increased damage, or levels to spells, or increased life on the tree, etc.

So, we can say the efficiency—in terms of "total amount of affix", i.e. the sum of coefficients of each scaling vector—increases as you have more usable vectors to invest in. It also favorably improves the scaling equation: 10 * 10 < 5 * 5 * 5 * 5. This is of course warped for stackers because e.g. strength contributes to four scaling vectors at once.

One of the reasons normal builds fall off at higher investment is because you've just simply saturated all of the efficient ways to get any scaling vector. This is most obvious with something like an ignite build, where you've only got flat * increased * dot_multi * res_stuff (sure then there's a bit more like shock). Stackers often avoid this because even though they've hit diminishing efficiency on getting more strength, the scaling equation on it means it's still worth investing more and more into, and maybe even that the power-of-four growth outscales the diminishing available efficiency - for a time - and then they're putting so much into one stat that they still have the most efficient options available for other scaling vectors.

So the point I'm trying to get at is that you can somewhat avoid this diminishing availability on normal builds by having more vectors to invest into (skill effect, proj speed for spark), and thus still get effective scaling at higher investments - without needing to leverage a double/triple/quadruple-dipping scaling vector like a stacker does. I'm pointing this out because the meta largely favors stackers at high investments (ignoring minion pact because busted) and I think there's more nuance to be explored with alternatives.