If you have the right equipment and supporting skills invested, sure. However, there are hundred world tiers of difficulty. Some will lose mobility, some will lose cast reach to climb that. Just as D3 speedfarm builds and push builds being distinctively different in terms of damage output and effort requirement to maintain their stance in combat.
In example, Greater Rift push builds in D3 will be slower and dish out high amount of damage in a small time frame or upon an intense area to clear the content and keep tabs on different variables.
All the while speedfarm builds of D3 will be highly mobile but their damage output won’t be as comparable, yet easier to play as it require less inputs and their effective cast reach is greater.
Also, you are skipping the detail that not every class gonna play the same effort-wise, let along different builds. Each class has its own fundamental designer, practice some trust at that. Do you honestly think that there will be only one cookie-cutter build per class with no repercussions? No, there will be many depending on the purpose, and this means build diversity on its own by end game tasks or effort and thought process in combat. Designing a class and a build are much deeper process than you thought.
You just feel like complaining about respec cost which is a thing that you can avoid completely if you play your own way and become aware of parameters of your build. On your fourth or fifth play through of the same class, you should have an idea about how to play your character from scratch and aware of its backdraws upon picking different skills. This called player’s learning process which naturally happens regardless of respec costs. Rewarding a commitment is simple as allowing player to avoid punishment which they can tweak at will.