Nope, you didn't have to use a Top 5 pick to get a 3 down back.
2014 Top Rushers
Murray - 3rd round
Bell - 2nd round
McCoy - 2nd round
Lynch - 1st round
Forsett - 7th round
2013 Top Rushers
McCoy - 2nd round
Forte - 2nd round
Charles - 3rd round
Morris - 6th round
Peterson - 1st round
2012 Top Rushers
Peterson - 1st round
Morris - 6th round
Lynch - 1st round
Charles - 3rd round
Martin - 1st round
2011 Top Rushers
Jones-Drew - 2nd round
Rice - 2nd round
Turner - 5th round
McCoy - 2nd round
Foster - UDFA
The only year in the last 5 where you see the Top 5 rushers in the league come from the first round was 2015. Peterson, Martin, McFadden, Gurley were first rounders, Ivory was a UDFA. But you've already stated 2015 was a fluke year.