If they leave it until late next year to 'come hard' I think it will be too late. The PS4 will have a huge lead by then (10m - 12m?) and Sony will just release bundle after bundle. Even if they blow everything they have before September, word of mouth will see them easily through Christmas 2015. I would say it's almost guaranteed that Sony will have a LBP3/RAC bundle and a The Order/Bloodborne bundle. I can also see them releasing a Driveclub Complete addition bundle with perhaps a couple of older games like Infamous: SS and TLOUR (again). MS need to do something much earlier than that.
The PS5 Exists.