That's the point: Bitcoin has rules - of which the limited supply is one - which can be changed by the miners, because they are not based upon some unchangeable laws of the universe...
Many if not most of Life's games, including money, have undergone rule changes, which did not make them an entirely new game, and did not lead to their immediate abandonment or demise...
Please learn how bitcoin works. If after the subsidy cut to 25 BTC a miner (or group of miners, or even 100% of miners) produced a block which had 50 BTC subsidy (or 20 trillion BTC subsidy) the rest of the network (AKA NON-MINERS) would reject that block as invalid. By the protocol rules the block is invalid. Miners can produce anything they want (even blocks w/ 20 trillion BTC subsidies right now) but if the blocks aren't seen as valid they are worthless. What some miner's do doesn't change the fact that the existing blockchain (w/ the existing rules and existing clients and existing miners who stay on board) will continue to exist. Nothing miners do prevents user from accepting the Bitcoin (defined by the rules that exist now), or prevent other miners from solving blocks according to the rules that exist now.
Hopefully, by the time the miners get too full of themselves to do that, there will be a P2P currency that doesn't involve "mining", and is based on rules that really cannot be changed.
The rules are merely a protocol; they can always be changed. It is impossible for them to not change. Even advocates of proof of stake don't make outlandish claims that the protocol can't be changed. A protocol is merely a set of "rules" that users agree to. If the users unanimously change the rules then they change. Period. If some fraction rejects the change there will be a fork in the network. The way mining works ensures the barrier to such a successful change is extremely high. However if 100% of users, 100% of developers, and 100% of merchants all agree to a change and all existing clients are updated to the change it is simply impossible to prevent said change. If some x% of users, x% of developers, and x% of merchants agree to a change the rest don't the network will fork into two competing protocols (sets of rules).
May the best rules win!Bitcoin is a consensus but it isn't just a consensus of miners it is a consensus of all users (stakeholders). As an example say some minority of miners create an incompatible fork (lets call it inflate-a-coin). Now MtGox could either accept deposits in the original Bitcoin or this new fork? Why would they change. A bunch of miners producing a fork of Bitcoin used by nobody, valued by nobody, and accepted by nobody doesn't really have any value does it? How will user's "see" blocks produced by inflate-a-coin? Well they would need new clients? Why would they upgrade? Why not just keep using the same client. If places they want to to buy and sell things use the existing protocol and their exchange uses the existing protocol, and their current software uses the existing protocol why would they suddenly change to an inferior coin?
So to have a successful change would require not just a super majority of miners agreeing to the change but also a
super majority of developers, and a
super majority of users (who would need to install the new forked client), and a
super majority of merchants, exchanges, service providers, eWallets, hardware manufacturers (someday if hardware wallets become part of the ecosystem). Even then it is a fork it just happens to be almost EVERYONE (not just miners) is using the new fork.
The idea that miners can simply force a change on the masses is as simplistic and incorrect as gold miners forcing people to pay good money for sand coins so they can sell more coins. Sure if everyone (or some sizable majority) in the world agreed sand coins were worth $1700 per oz then it would work, otherwise the gold miners would simply have lots of unsold sandcoins. Likewise miners trying to force a change in the minting rate without global acceptance would simply have a lot more useless worthless alt-coins.
Which is worth more; 25 BTC per block of "real Bitcoins" valued at $x or 5,000,000 BTC per block of "fake Bitcoins" valued at $0?