The deflationary nature of bitcoin is by design, yes, but the thing with cryptocurrencies is - if it's somehow unsucessful, people can either ditch the currency and make a new one, or if most of the miners agree by consensu, even change the bitcoin protocol with its deflationary nature itself.

For example, if the economy will be in such a terrible state thanks to the deflation, somebody can do a fork of bitcoin that will not have halved mining reward (and thus not having finite number of bitcoins in existence), and if the miners will jump to this new version (and probably clients, too, not sure about that), this new currency will be used as easily as the old bitcoin.

If I am wrong technically, please correct me.

The nice thing about bitcoin is - nobody enforces anything.