If you trust the financial system to store your funds, or Dropbox to store your files, or YouTube to host your videos, then you don’t need to use less efficient decentralised versions of those services. But if you are planning to commit financial crime, store illegal downloads, or host pirated videos a decentralised version of those services becomes much more appealing. That’s why bitcoin, for instance, has become the currency of choice for online drug dealers and cybercriminals demanding ransoms to restore hacked data.
Building a system to be trustless comes with huge externalities; the true cost comes elsewhere in the overall system.