It is unquestionably a bad design call as the proposed changes sit in direct contradiction to the talent design philosophy they have champion since MoP. Ironically, this very talent design philosophy, which has been built from the ground up with the concept of talent switching as a response to the varying encounters the player is faced with in the game world, has actually become more entrenched in Legion. This is evidenced by the fact that certain fundamental aspects of each spec's toolkit, such as appropriate AoE abilities or burst dps cooldowns, have been re-allocated to the talent system. Moreover, there has also been a clear and conscious effort on the part of the developers to have single-target, AoE and cleave talents actively compete with one another on individual talent rows - thereby promoting the idea that one should switch in-line with the requirements of the encounter at hand, should one wish to play their class to its full optimal potential of course.
What you've seen from certain proponents of this change within this thread is a desire to minimize this level of talent swapping, in order to return to an era where we each selected a standard build, usually reflecting a satisfactory middle-ground for each spec, and stuck with it - the consensus appears to be that subscribing to these established cookie-cutter builds reinforces class-identity. That's a perfectly valid opinion to hold, I guess, and I have no issues with players expressing that desire; however, it does not alter the fact that these proposed changes sit in contradiction to the system they have built, developed, and iterated upon over the last three expansions. I suspect they are really going to need to heavily re-assess the talent system if they do wish to go down this regressive line, but it is surprisingly late in the development of Legion for this jack-knife in their philosophy to suddenly emerge, and even later for them to make the necessary changes to relevant talent rows across all specs to ensure that the player-base isn't unduly penalized for it.