PewDiePie is one of the richest Youtubers, who has little to no skill outside of playing games and acting hysterically to entertain his young audience. If he were to delete his youtube account he would have nothing left to sustain him. While he is still young, his riches most certainly won't last forever.
PewDiePie is a comedian and he knows exactly how to play to an audience. He will absolutely not delete his Youtube account. As long as it continues to generate the revenue that it does, he will continue to do what he does best, which is entertain. He has made a fortune and that gives him a lot of reasons to continue. It was reported that PewDiePie grossed over $15 million this year. In my opinion, that is reason enough for him not to delete his account. I believe it is a publicity stunt and Youtube could very well be a part of it as a marketing campaign for their viewership and monetary gain.
Youtube sensation, PewDiePie is threatening to delete his account when it reaches 50 million subscribers. That is likely nothing more than a stunt to get more subscribers. More subscribers means more money. When he reaches 50 million he will likely come up with an excuse for not shutting down his channel. He will lose some credibility; however, in the end he will likely end up with more subscribers.
No, PewDiePie will not delete his Youtube account when it reaches 50 million subscribers, because he likes the attention. There is no person or group that goes to that length to have that many followers, just to completely give up on it as soon as the person reaches the goal. They are more likely to want to continue to post videos for comments and likes.