I actually agree that a "let players access everything, but let players pick what they get and when" system could work. As for the theory: Any skill tree without resets is unlikely to have meaningful decisions without the potential for screwing up. Combine that with DIII attempting to be legitimately hard on higher difficulties, and you can't get away with giving players permanent choices to make non synergistic decisions, and there's no way to get remotely close to the theoretical ideal where a random or semi-random allotment of skill points can be as effective, in its own way, as an actually thought out and calculated plan. Of course, you can have permanent suboptimal choices and still allow all players to access all content (see Fallout 3 and New Vegas), but that mostly relies on making the optimization floor significantly higher than the difficulty ceiling (i.e. random build with no coherence still kicks more ass than That One Boss).