Instead of advocating for making it optional, why not advocate for streamlining. Moving some MSQ quests (particularly in 2.1-2.55) to side quests is probably the best option. Reason I say this is it leaves all that content in game, but you no longer have to complete that specific facet. Cause in each xpac and the base, there are some MSQ quests which are definitely more flavor and doesnt drive the actual plot forward all that much. Theyre more like side events that occur inbetween major events. Stream lining like this would be a viable option if done carefully. MSQ is still there, you still have to complete it, though its not as long and more streamlined, and the moved quests still exist if you want to experience that content. It's a bit of both sides, MSQ remains in tact broadly, and newer players dont have as much to go through. its a decent compromise.