Plenty of players have complained in recent years about low hits. Most would rather take a high hit (and a possible concussion) when defenseless than a low hit (and a possible knee injury).
Collectively, the players have not yet made a push to expand protections for defenseless receivers to include low hits. Per a league source, the NFLPA plans to study after the season ends all data regarding knee injuries before deciding whether to make a push for the establishment of a strike zone.
Even if the players don’t ask for it, the league may do it anyway. Albert Breer of NFL Network has reported that the Competition Committee will consider in the offseason protecting defenseless receivers against being struck below the thigh.
The ability to hit opponents low helps smaller players take out bigger players. Over the past four decades, however, the NFL has periodically and systematically restricted the situations in which a player can strike another player low.
As Breer points out, the lower limit to a strike zone wouldn’t have protected Patriots tight end Rob Gronkowski, who was running in the open field when hit low and necessarily not defenseless. Of course, Gronkowski wasn’t protected against a high hit in that situation, either. But when he’s running in the open field few defensive backs can deliver a high hit to a guy that tall.