I kinda like this idea.
It makes no sense that you have to display mastery of a level *before* the game will allow you to play it solo, at your own pace. I do not understand the reason this change came about, and is the opposite of what should have happened. New players will find the game chaotic and hard to learn when thrust into multiplayer. Other players will just carry the burden and do everything, and noobs won't be able to learn like that. I played every mission solo before I ever did multiplayer, and it made me a better player, rather than cannon fodder running around like a headless chicken.