There are many variables on how this could play out. The biggest one being will airlines that fly internationally have to accommodate pilots over 65. If they can not offer accommodations on the grounds they don’t have domestic only jobs (pretty much all planes at all majors except maybe the 717 and 220 at DL fly to countries that still have 65 as the law) then hiring may not be as affected.

can’t speak for others, but as far as DL goes, my guess is hiring will slow to 50-80 a month assuming they are forced to accommodate 65+ pilots. With the caveat that if the economy slows like it did last time the age was raised things will stop and hopefully not move backwards