change to implement.

Allowing the defense 10 or maybe 15 seconds to match a substitution would probably require an additional "substitution clock", with someone designated to start that clock when the last offensive player steps onto the field. It could be done, but that would be quite a bit more complicated than just saying "the offense can't substitute if there are less than 25 seconds left on the play clock, and if they do substitute before that time, then they can't snap the ball until there are 10 seconds left on the clock" (or whatever the numbers should be).