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.
It depends though; I wouldn't say "hardcore fans" as there were a lot of people mining with a lot of GPU's before the ASICs got released.
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)
It does not make sense to join that "team". Toomin won't even go with Bitcoin Core 0.12 (
libsecp256k1 ), but rather 0.11.2 (i.e. it is possible that he is unable to apply the changes himself). Some might attack me for stating this, however I only said that it is a possibility. I can't find a good reason for which the changes should not be included in this release.