That would be a hard fork.
Yes.
Miners don't vote on BIP all users do.
Imagine the situation when top-10 mining pools will accept this protocol change in 4 years from today.
There will be split of network in ~2018.
I think that the majority of users will follow the pool-owners
SPV-clients do not have right to vote at all about protocol changes. And the number of full nodes decreasing.
I can't really see that your proposal would ever be accepted by a majority of users.
I think that it is possible. I do not put 1 satoshi for this case of future events, but it is case of future.