that you are at least taking time to research into Bitcoin. :)
I need to correct a few misconceptions you have.
First, about "beneficial in the long/short term" I was saying beneficial either way. Long or short term. Doubling the 21 million limit aka inflating a currency doesn't seem beneficial in a long or short term to people holding it.
Next, about a protocol change. It's not agreement of a majority of miners. A successful protocol change requires everybody - merchants, miners, and users - to upgrade/switch to the changed version. Any protocol changed version is NOT compatible with the old version, i.e. not the same coin. The people that switch are on a new system, although they can use the blockchain from the old system if they desire, to determine who has what coins. This comment explains it well.
Next, bitcoins don't get harder to mine as you near the 21 million limit. The "difficulty level" of the computational mining problem is adjusted so that bitcoin creation roughly follows this inflation schedule. As more or less computers work on the same problem the difficulty is adjusted so that the same coin creation rate happens regardless of total computing power.
Next, you mentioned "checkpoints" for the system. From what I understand there is a checkpoint placed in every released upgrade (non-protocol changes) to the Bitcoin client software. This can be thought of like a fire extinguisher. It's nice to have, but you hope you never have to use it. If you do have to use it at least it can help limit the total damage that can be done, in this case from a successful 51% attack. Nobody has the ability to roll back transactions to a prior point. You would have to agree/opt-in to going to that checkpoint.
Next, regarding nullifying transactions, that happens with a successful 51% attack. Please see my comment here regarding that.
Lastly, you're talking about the likelihood of people switching away from Bitcoin to something else, possibly better. There have already been other crypto-currencies launched. Some have been around many many months now, and nothing is even close to Bitcoin. At some point in the future there may be something that actually is better than Bitcoin. That's perfectly fine. Bitcoin is governed by the free market and people can switch as they desire. Same with gold. It's arguable that dollars might have been a better money system than gold if Congress controlled them and they were never inflated, right? And people could have switched or held gold as they pleased.
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