We don't "need" almost everyone for any change. That's what the minority would like to think but no, if 75% agrees to kick out 25% away. Then 75% have come to a consensus to get rid of the armful 25%. End of story.
Why 75% then? Your argument makes no sense from this perspective: 51% resembles the majority as well and you could time the hard fork with a difficulty adjustment (this wouldn't be a problem). Then the 51% would have come at consensus and got rid of the armful 49%, right?
I used 75% as per Classic threshold but you are right, any % can make it for a split. Consensus is all about compromise. If one group is unwilling to do ANY compromise then the only thing they should expect is a letter of divorce from the other group, regardless of how much % they represent.