One attribute wont necessarily do more dmg for everyone. It might depend on other aspects of your character.
For example, lets imagine:
Str = +% dmg
Dex = attack speed
Int = proc chance
Will = Lower resource cost
(not exactly how I would design it, just trying to keep it a bit simple for the example)
Attack speed from Dex might be balanced to give more dps increase per point, than the +dmg from Str. But Attack speed automatically comes with the cost of increasing resource cost, for some skills (and being fairly useless for skills with CD). So attack speed would win for some builds. +dmg would win for others.
Same with Resource cost reduction. It might be better than +dmg for the most expensive skills, obviously useless for generator skills, and likely not great for cooldown skills.
Proc chance, same deal again. Might be stronger than +dmg for skills with proc effects, and useless for other skills.
(which again, is why this is ONLY an example. Each attribute should offer multiple effects, so even if you dont use any skills with proc effects, Willpower would still give you other benefits)
Sure, each build will have one of these attributes be the strongest one for them (though for dex and willpower, since attack speed and resource cost interact with each other, they might fight for the top spot, depending on your gear etc.).
But at least different builds would have different preferences. Which is much better than what D2 and D3 offered.
We cant really avoid that most things in an RPG can be solved mathematically. Nor is it necessarily a problem that it can be solved - solving it kinda is the game. The goal should merely be to offer different optimal solutions to different equations.