The fork has nothing to do with money supply, it has to do with whether and how to relax a constraint on transaction velocity before that constraint becomes a limiting factor of Bitcoin's utility. This fork is not inflationary or deflationary.
The fork is exactly about money supply. The primary argument used by those in favor of Gavin's proposal is that the fork is needed in order to make bitcoin more inclusive. If we make concessions to these demands now, what is stopping the same people from insisting we create 5 million bitcoin to airdrop into the wallets of some 3rd world country? When getting "mass adoption" is the primary focus, you end up with things like the dollar. I don't want mass adoption; I want sound money.
The slippery slope notion?
We are all likely in agreement that the fundamental purpose of Bitcoin is sound money. If not, then another thread for dealing with "first principles" is needed.
One of the elements of sound money is its utility as a "medium of exchange".
This is what we are addressing with this hard fork.
The thing that stops the same people from destroying the sound money principle (and your example of 5 million more bitcoin airdrop) is the "store of value" element.
My last few posts here in this thread have been an attempt to remove the confusion between these things. We can advance the "medium of exchange" utility, without harming the "store of value" utility at all. This particular hard fork is not a negative impact to the store of value. If anything, an increase in the velocity of money improves the store of value utility through increased liquidity.
I'm of the opinion that "Mass adoption" is a pipe dream (the market agrees with this). I think it is decades away, if ever However, I am working toward making it possible, and I am opposed to things that would make it increasingly impossible.
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I also agree with your idea that doing something "because Gavin said so" is the height of laziness and foolishness. I think even (perhaps especially) Gavin would agree with that. His role is more of a curator of ideas and he should not be expected to come up with everything.
There are a whole branch of folks here that disagree with this. They think that we should do just whatever Gavin proposes. They think that "ossification" risk will kill Bitcoin. They are as wrong as the people that think that we should not do something because Gavin proposes it. It should not be about the personalities here. Bitcoin is more important than that sort of political nonsense.
It is the responsibility of all of us out here in the cheap seats to add some peer-review rigorousness to the process. Its a messy job.