One nerf could be enough, Bolt fletching was affected twice in one update. What is the return on this "upgrade?" The ability to semi-afk train. Some of us prefer higher efficiency with more effort - why do you think that people use Sawmill or Teaks instead of Ivy? Why should those more committed to their training be penalized? What is the reason why Jagex encouraging laziness?
Multiple skills can be taught at the same time, making it more efficient than just one. Arguments that one skill should always be taught at any certain time can be made by people in banks who are left wondering why it's tedious and slow. Either that, or they think that the best way to train multiple skills at the same time includes, for instance fishing, and then making use of the fish that is caught for cooking.
It's not against the rules, or even the nature of the game to work in multiple areas of skill at once. Zarfot is a famous figure for his efficiency in acquiring each skill, was able to get 200M Fletching employing this method.
It is impossible to learn another skill while also performing other tasks. Alching is a close match, but it is restricted by the Standard spellbook. There is no Ourania teleport casting, no casting in a Monkey form, slow casting time (MUCH faster XP than Bolt's fletching), constantly switching to buy rs 3 gold the Spellbook tab.