It was most definitely expected, this the reason why most big budget games are shooters nowadays , they sell a lot and make loads of money.
Although many still believe that MK 8 will overtake it, I think it's impossible, Destiny by design is a game which will get better over time with the addition of content ( which is also it's major flaw ATM ).
Further to keep things in perspective watch dogs ( at 3 million + sold as per VGC) is showing better legs that MK 8. and that is ugh... ( IMO :p )