Sliding is when no one is around you really. When you slide in traffic or too late, even in space, you get rocked and it's almost a guaranteed injury/concussion. I actually think the sliding only makes sense in very specific circumstances because it actually puts you in worse position. I think what people should be telling him is to be judicious about when he tried to advance the ball on the far edge of the defense. If you can get just outside the tackle box, sure, get some space and slide. But if you're right at the end, either hustle out of bounds faster, throw the ball away hurriedly or find some way to dive out of bounds because you're less likely to get the killer head shot when doing that than when sliding.
People keep talking about sliding but this is the SECOND concussion Robert has gotten from a (IMO) cheap shot to the head when sliding. Defensive players are consistently trying to hit QBs as low as possible NOT because they initially mean to just take out legs from under them but because they are trying to get cheap hits on the QB's head. There's no reason to launch that low on a player who is already on his way down.