Right now, yes, it would be extremely difficult to get everyone to change. But what about in the future when the situation is different. Could a majority of bitcoin users/miners be convinced to change the 21 million cap? If the leaders of the bitcoin community all supported a change as the best solution to a serious problem that was haunting the currency/economy, is it still impossible that a majority would adopt the change?
If a significant majority felt such a change was in thier interest, then they culd make that change, aad everyone else would prett much just have to go along with it. It's easy to see this as a negative, but actually experienc shows it's a positive. If something turns out to be badly wrong with bitcoin, it can be fixed. And the vast majority of biotcoin users understand that being able to rely on the basic assumptions to say the same is necesxsary for a currency to have an yvalue.