I think it has significant impact.
It's a stupid rule. But if it passes, I think in the real world it will barely be noticed. Even hurry-up offenses almost never get the ball snapped in 10 seconds. And defenses won't be able to substitute in 10 seconds either.
You are right, it's rare that the ball is snapped within 10 seconds, but it's not rare that the offense has hurried up the officiating crew enough to the point where the snap could happen within 10 seconds. It's the threat of the quick snap that eliminates the possibility of a quick sub on D. Quick subbing to address a matchup issue is a big deal against the hurry up. That's the bread and butter of it. Find the matchup and exploit it without giving the D a chance to adapt. It's smart.
However, my line of thinking is that since nobody technically controls the ball between tackle and the point the ref spots the ball for play, neither team should be able to influence the pace in which the refs do their jobs. Hurry up offenses have done a good job at getting the ball in a position to be set quickly by shoving the ball in the refs chest and rushing to the line. That's fine, they are doing what they are allowed to get away with, but the D doesn't have an equal opportunity to slow down the ref without doing something that could penalize them (sit on ball or sit on tackled players).
I'd prefer it simply be a point of emphasis for the refs to do a better job at setting the ball consistently without influence from either team, but this rule will take the possibility of them 17ing it up and catering to one team out of the equation. I'm okay with it if it passes, but if it doesn't I hope referee pace between snaps is a matter the NCAA focuses on.