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)
not but they will continue to keep both of them. Is not problem at all for all of us to continue to have the same coins in both chains. This will confuse many and many will lost the trust in bitcoin system and that mean economical disaster for bitcoin ecosystem.
Is simple if a hard fork happen without consensus we will have.
1. Bank run from cloud wallets and exchanges
2. Collapse of bitcoin value
3. After that collapse of bitcoin mining
4 And at least to parallel bitcoin that all can trade for each other
There is no possible 90%+ consensus with so many people and different interests here. The 10% could easily be altcoin lovers who dont mind hurting Bitcoin a bit or have different reasons for capping onchain useability. I dont know how you want solve this problem then...
anyway i dont think ever possible to a hard fork to happen in the next months.
Apparently some of the Classic supporters seem to think that it is (safely) doable within a single month. This makes it obvious that the main idea behind the fork is a "upgrade" to the ecosystem. 
Switch was doable in one day, one month is enought but more time wouldnt hurt either this I agree. Push Bitcoin Core to compromise with 3 or 6 months grace period then...