There is nothing technically difficult about making this change. Edit a few lines of code, upload a new version, make sure everyone installs it.
However, it is extremely unlikely that any respected developer will ever try to make such a change, or that enough users will adopt it if it is offered.
The reason is that people who hold bitcoins and don't get rid of them like hot potatoes, do so with the understanding that every BTC they have entitles them to 1/21M of the total eventual worth of Bitcoin and that there will be no unplanned inflation. They have confidence that the generation schedule will not change, because it has never changed so far. And people who would consider condoning a change, know that by so doing they will create a loss of confidence for future generations who will have this precedent. And, everyone knows that everyone knows that changing the schedule will create a loss of confidence, giving even more confidence that there will be no change as nobody would condone it. And so on, ad infinitum.
In short, as long as the generation schedule is sacred, it acts as a Schelling point for the confidence in long-term projections. Changing it even once will make this confidence collapse completely.
If people want an inflationary decentralized cryptocurrency, they will have to create an alternative currency and leave Bitcoin out of it. The inflation rate can be set in stone, or if they think a mutable schedule is good, they can specify on what terms it can be changed, so it can be safely changed without destroying anything.