MRIs and CT scans cant detect concussions. You mainly get an MRI or a CT scan to rule out brain hemorrhaging.
I'm sure the NFL will re-evaluate the concussion protocols based on Tua's last two games, but short of declaring all players whose head slams the turf out for the rest of the game until an MRI can be given, I'm not sure what else they can do. A third party doctor already gives an evaluation. But that evaluation means little without an MRI that can actually see if any damage occurred.
I'm sure you could mandate a booth doctor whose job it is to declare a player out because of potential injury, but do you really want to give that much power over a game's outcome to one person? It's a violent game. The NFL has had many opportunities to change the design of their helmets and they have refused most of them because they are very expensive and not very eye appealing. That may be more of a problem then their concussion protocol.
The NFL has done a lot of pretty good work with the helmets. The reason they haven't gone with a soft helmet is because they haven't done that well when the player hits his head on the ground. The hard helmet has less friction and does a better job of keeping the neck stable, which reduces catastrophic neck injuries.