Re: Suggested change to future mining rate.

Another option is to make it 100/((n/4)^2) with special adjustment to 50 for n=1, this still keeps the first 8 year the same as today, thus limiting short term effect, and also guarantees a finite total number of BitCoins.
If wanted some other adjustments can be made to make the total 21,000,000 but still have a heavy enough tail.

If you release a version of the Bitcoin client with this change made, and 51% of the network starts using it, then congratulations, you've changed the rules.

Not really, the 49% will just keep using Bitcoin with Bitcoin rules.

What happens when the longest chain doesn't match the client's rules for what a chain is supposed to look like? If I tell my client that, say, new generations can only be 5.0 BTC, what will it do?

Bitcoin rejects invalid generates (and other tx) regardless of how long a chain they are in. This is why the >50% attack is double spending or stopping tx only not magic creating money.

You can have your chain with as many or few people as you like, it just isn't Bitcoin and no Bitcoin client will ever accept it.