OK, so here's a better proposal: with 15 seconds or less on the game clock, only players on the field may call a timeout, not a coach on the sidelines.
The offense will see that a defensive player is calling TO and not run the play, thus eliminating the risk of injury. There's also the sportsmanship issue - defensive players on the field will be less likely to try and hose the guys lining up across from them with truly weak-sauce stuff. On the other hand, if they need to call TO legitimately (or even to ice the kicker), they still can. It's just that the coach, 75 feet away, can't do it from the sidelines at the last possible second.
Sound like a better proposal?