Not useful to law abiding citizens, exactly. There's plenty of people who are not law abiding citizens.

The scarcity is debatable because of forks. When a coin forks, then people need to settle which of the forks is the legitimate coin, and which one is the fork. There are several institutions and people who have influence over this discussion: Miners (They decide which fork to mine on), Exchanges (They decide who gets the old name and ticker symbol), platform landlords (they have significant ability to steer discourse), media (see previous). If enough of those came together, and decided that there should be 21 trillion bitcoin instead of 21 million, then there will soon be 21 trillion bitcoin.

While my example is far fetched, crypto currencies being forked to subvert their rules has already happened. For example, the ethereum/ethereum classic fork. The Ethereum blockchain got forked to roll back the hack of the DAO. Nothing the Hackers did violated Ethereum's protocol, meaning that technically, they did nothing wrong. An example of a political decision which is harmful to the long term utility of the crypto currency would be the Bitcoin/Bitcoin cash fork. Bitcoin has a block size limit, which leads to a transaction limit of 7 transactions per second. This limit absolutely ruins Bitcoin's potential utility, because even relatively small economies would be utterly choked with a 7 transaction limit. Still, miners profit from a tight transaction limit, and other institutions aren't bothered by it, so they decided that the fork with the transaction limit was the legitimate one.

In conclusion, the 21 million bitcoin limit will remain as long as it is useful to the influential figures in the bitcoin ecosystem. See above for a rough listing of who those people are.