the ID service in Matrix is optional, and should be called an 'ID mapping service', really. All it does is to map 3rd party IDs such as email addresses to decentralised matrix IDs. Currently it is a logically centralised service, and this is a major shortcoming: in future we intend to decentralise it as much as possible. This is Hard however, and we're kinda hoping that a decentralised equivalent to Keybase will come along and do the work for us :) Until then, you don't have to use the existing centralised thing if you don't like it :)