To increase the total supply of a decentralized currency built on a distributed network of self-sovereign full nodes, you merely need to convince every single person operating the said nodes that possibility of mimicking the traditional financial system on the bitcoin blockchain is worth considering and that inflation leads to hastening of economic growth, and that they should give up part of their wealth for the common good. Rational actors will surely refuse such an offer because no one sane would agree voluntarily to become a person who is gradually being robbed by whoever will be in charge of the bitcoin printing press.

Scarcity is a core characteristic of bitcoin that was set by Satoshi themselves. Had Satoshi come up with a smart algorithm of inflation adjustment independent of human decisions and implemented such an algorithm initially, the scarcity of Bitcoin wouldn't have existed, and we would have been using inflationary Bitcoin, but still in a censorship-resistant, unstoppable, decentralized way as we do nowadays with scarce Bitcoin. But Satoshi decided that, in the absence of such an algorithm, the most prudent solution would be to make the total supply fixed and known in advance.

Keep in mind, no one forces you to use Bitcoin. If you don't like bitcoin being disinflationary, you can always fork yourself off chain and spin up your own version of inflationary money.