A significant percentage of the population can be manipulated to support anything, but bitcoin is a consensus system. Hitler did not manipulate 100% of the population to support him. There were many who refused, and who actively acted against him. There was not 100% support for any of the wars in Iraq or Afghanistan. All of those wars had individuals who publicly spoke out against them.
What percentage need to support it? Is a majority of full nodes sufficient?
No.
Bitcoin is a consensus system. You need 100% to change.
If 2 people refuse to change their software, then they will continue to communicate with each other as peers and will continue to operate the old protocol. They will continue to call their system "bitcoin", and their system will still be the original "bitcoin". Any other system will be an altcoin.
Now if the majority decide to call their altcoin "bitcoin" as well, there will be much confusion, and a lot of complications. The more people that choose to switch, the less confusion there will be, and the more likely that the new system will not fail. At that point you'll have most people calling the new inflationary system "bitcoin", and a set of idealists calling the old system "bitcoin" and using whatever derogatory slang they like for the new system.
Think about it this way...
If Satoshi had decided to call his invention "dollars" instead of "bitcoins" (and left everything else in the design exactly the same), would the fact that the vast majority of people were using a different system called "dollars" have meant that his "dollar" system would automatically switch behavior to be the same as the "dollar" system that everyone else was using? Obviously not. There would just be a lot of confusion with two different monetary systems both called "dollars".
Here's another way to look at it. If the creators of dogecoin had decided to name their system "bitcoin" (and leave everything else about dogecoin exactly the same), there would obviously be a lot of confusion as people tried to talk about exchanging legacy "bitcoins" for "new bitcoins" or vice versa. However, the fact that the new "bitcoin" (dogecoin) (an inflationary altcoin based on the legacy bitcoin protocol) is popular and has many users does not mean that the original "bitcoin" system has become inflationary.