the most possible scenario is the rest 25% to keep running the old bitcoin system and to change the mining system and ban asic miners. And i am very sure that for many months we will have two crap coins with no value at all and with 21 supply coins each other.
Why would the remaining miners agree to make their equipment invalid?
Your right, changing the PoW algo would be BitcoinCore decission, not miners decission. So Instead of about 20% support it would be changed to only hardcore fans wanting to back old Bitcoin with GPUs. Then the 20% of ASIC miners eighter switch to new Bitcoin rules, start supporting other SHA2 coin or sell mining equipment.
And about "we will have two crap coins with no value at all and with 21 supply coins each other", I dont think there is much incentive for the 20% minority to continue with 1MB blocks after the 28 days grace period, but let see who is right (if there really become over 75% support for the 2MB modification)