It isn't. Who said that?
There is a difference between "we need a hard fork to increase the block size" and "Gavin's plan is the way to do it, (which ever plan he settles upon)"
Most all the devs want a hard fork to increase block size. Very few are on board with Gavin's plan.
Anyhow, it looks like the Bitcoin Core hard fork will be more likely to progress from gmaxwell's BIP, and the XT fork from Gavins perhaps.
They may remain compatible until there is a block that one would process and the other wouldn't.
You can include me in that contingent: we do need a hard fork to increase the block size (and only for what that will achieve, buying time). Very few people are debating that now, and I myself have been aware of the scalability issue for almost as long as I've been interested in bitcoin.
Re: which designs will be implemented on which fork; I thought Gavin had decided against the hostile fork?
I would rather see voting by miners done in the future for changes to be made in the future, than see guesses made today applied far into the future.
That sounds sensible, but it also sounds like you're signed up to Greg Maxwell's prospectus on the issue. I like the idea of dynamic sizing, and of flexibility to make decisions only once that decision is required, but arguments have been made that this creates a perverse incentive for the miners to collude on the vote. Not to say I agree with that, but I haven't fully decided yet. This debate needs a really long time to be adequately digested. Influential people in this community arguably don't understand the system we have now for what it is, the idea that this debate is somehow ready for everyone to make an informed decision is not sensible.