Yoshida literally and openly said that they pursue simplification to make balancing future fights easier. I don't remember now in regards to what he gave that answer (probably as to why there are no gear varieties at the same item level anymore), but the answer was general enough.
That's why the stat distribution was removed, why elemental affinities were removed, why the class/job system was abandoned (and they didn't even have the courage to actually remove it leaving the trashed system behind), why the hunting log was abandoned. They make a lot of systems then abandon or remove them because they turn out to be too much of a pain to develop in the long run. Even leve quests for disciples of war. They abandoned it because it was too much work to give them flavor. Because writing "A rise in humidity increased the marlboro population. Nip the problem at the bud." is apparently too hard.
As for the role skills, I think you missed the point of role skills as stated by the developers. Their purpose was to make sure that every player had every necessary skill at their disposal without having to level other classes. Because things like Swiftcast always were "mandatory" for all related classes. That is large part of why this system was a complete and utter failure. Because of how limited the slots for them in comparison to the skills, people ended up picking four or five per class that everyone and their mother will have leaving the other skills behind. At that point this system is strictly inferior to simply GIVING every class the toolkit that they need to begin with.
It never was made for customization. Cross-class skills were made for that...but the choice of skills in it were simply flawed. Majority of the skills in questions simply had no utility whatsoever outside of classes that already had equivalent skills to begin with. Heck. There was even a warriors combo starter skill if I remember correctly...without any combo compatibility or purpose.