What would happen if the blocksize were increased to 1 GB tomorrow? Pretty much nothing. Miners will always be able to create blocks less than the maximum blocksize.
What would happen if the blocksize were decreased to 1 KB tomorrow? Bitcoin would come grinding to a halt.
Too small blocksize = death to bitcoin.
Too big blocksize = non-issue.
I'd rather see the blocksize too big than too small.
That is nothing like Gavin's proposal for good reasons. To answer your question of What would also happen if the block size were increased to 1 GB tomorrow is the introduction of new attack vectors, which if exploited would require intervention to resolve by miners, and development.
It is not enough to design something that works, we must also design so that it does not become more fragile.
Why not strive for a dynamic limit that prevents the need for future hard forks over the same issue?
Gavin's proposal is "the simplest that could possibly work".
I'll argue that it is just too simple, and too inflexible.
This proposal may be opening Bitcoin to new types of coin killing attacks by assuming that anti-spam fees will always be sufficient to prevent bloating attacks. Consider that the entire value of all bitcoin is currently less than 1/10th of the worlds currently richest man, and that man has spoken publicly against bitcoin? When you include wealthy institutions and even governments within the potential threat vector, the risks may become more apparent. We can not assume Bitcoin's success, and then predicate decisions necessary for that success, on that success having been already accomplished.
If Bitcoin has to change due to a crisis, it ought at least be made better... so that the crisis need not be revisited. (Hard forks get progressively more challenging in the future). Design for the next 100s of years, not for the next bubble. Fix it right, and we fix it once.
Designs ought to have safeguards to avoid unintended consequences and the ability to adjust as circumstances change.
My suggestion is that perhaps we can do better than to simply assume an infinite extrapolation, when there exists a means to measure and respond to the actual needs as they may exist in the future,
within the block chain.
50% may be too much in some years, too little in others. The proposal is needlessly inflexible and assumes too much (an indefinate extrapolation of network resource). Picking the inflating percentage numbers out of a hat by a small group is what CENTRAL BANKERS do, this is not Satoshi's Bitcoin.
I'm not convinced a crisis necessitating a hard fork is at hand, but I am sure that the initial proposal is not the answer to it. I look forward to its revision and refinement.