We've discussed here why the arguments against Bitcoin Unlimited are fundamentally self-defeating. In that same vein, I think the following is a good question to ask BU skeptics: "what exactly is the alternative to 'allowing' the block size limit to be determined by the people that actually make up the Bitcoin network (i.e., the people who, whether we like it not, actually possess the ultimate power to do so)?"
I still haven't gotten a good, straight answer to that question, but after a few of these conversations, I sort of get the impression that their real answer is that they view miners as the block size equivalent of alcoholics. Sure, they have the power to drink (set their own block size limit), but if they're smart, they'll accept that they can't trust themselves to exercise that power responsibly. So I suppose that presents two options. They can either swear off drinking entirely (no block size limit increase ever), because they recognize that as soon as they have that first sip (mine that first 1.1 MB block), it's liable to send them off on a bender where they drink themselves to death (mine the network to death). Or they can outsource the decision of whether and how much to drink to a trusted third party: "I don't trust my own judgment when it comes to booze (block size limit settings), but I do trust my buddy Greg. Obviously he'll never let me have hard liquor (substantial on-chain scaling via hard forks), but if he gives me permission to have a small glass of champagne on a very special occasion (a modest increase in effective block size via SegWit soft fork), well then it's probably ok."