The preoccupation regarding Brazilian players' attitude is very understandable, there is a whole generation which was lost because of this.
But anyway, Neymar is a very hardworking young fellow. And if you take Ronaldinho, Ronaldo or Adriano's case, they all started to fall and lack motivation in their late 20s. Apart from Adriano (which is a unique case, IMO), they all worked hard at least till they were 27/28.
So, even if this happens to Neymar (who so far has been very professional - the kid has played more than 70 games last year), it will take around 7 or 8 years. He has a lot to give yet, and a lot of room for improvement.
People are afraid he could be the next Robinho, but it is very clear that he's better. He's more objective and has much, much better grounding in passing and shooting. Also, he's not a selfish player. Robinho's talent was more raw, with a lot of dribbling, but poor passing and shooting.
I'm pretty sure he's the next on the line after Messi and Ronaldo. But anyway, he's not leaving Santos until the 2014 WC, and he could even stay after that (he's earning around 3M/month in wages and sponsorship).