1) Its pretty much a non issue because anyone currently employed will have 1500 hrs in 3 years.
2) An ATP is just a type ride. At many airlines, FO's and captains pretty much do the same maneuvers and approaches. The only difference would be learning the callouts.
At the time when the ATP is required, EVERYONE should be an ATP if they work 121. There will be plenty of time to prepare and get certs done. I think the issue should be that anyone working the line for 121 should have an ATP, not, anyone hired should have an ATP.
The difference is that a new hire would need the time to qualify, but could potentially take the ATP/Type ride at the conclusion of sim training.