Here's the thing though, more experience Mario gamers (like myself) shouldn't have any trouble anticipating the challenges a level will throw at you. If you lose, say, one life due to your own personal error, then the Wii Demo will be basically mute in your play through since you are experienced.
On the other hand, a less experienced Mario gamer (new or otherwise), might get frustrated after losing more then two lives. I think eight lives is a bit overkill. The gamer in question might give up playing the game and never come back to it.
NSMB DS's later levels were a bit on a difficult side, but it was nothing staggering in difficulty as, say, SMB3 Dark Land (I still make timing errors in those levels before the first mini-castle). If the videos are any indication, NSMB Wii's later levels should be a sight to behold.