I think he'll ultimately finish 4th. I see him passing Bossy sometime next year and staying at #3 for a time period that exceeds Bossy's career number of games. Bossy's career ended early though, so he practically played in his prime years for his entire career and only played a limited number of games in his "declining" phase. Additionally his last season was 86-87 when there were an average of 3+ more goals scored per game than today's era. If Crosby plays into his 40's and the league scoring rate remains static, it will be very difficult to not have some "declining" years drag down his career average. 1.5 PPG is almost impossible to maintain in this era and very few players have achieved that over the course of single season since the early 90's.
That being said, if you normalize the scoring of each player to that of the league goal scoring average during their respective era, Crosby will be closer to the number 1 player in that category, Lemieux.