Signal is released under the GPL, meaning that if something did happen to shake user faith (data breach, data mining) a fork pointing at different servers could be instantly created with complete feature parity.
Every traditional barrier to user migration is removed. Features are the same. UX is the same. Compatibility is the same.
That's a precarious line to toe.
Also, Mastodon is built on open standards that have several competing and compatible implementations (GNUSocial and Pleorma are the main two), which are run by their own maintainers.
Yes, Mastodon is inter-operable with OStatus and ActivityPub. However I do not know of any significant user base that interacts with Mastodon through an alternative platform. Given the will, Mastodon could implement its own extensions to ActivityPub and break compatibility, with most users unaffected due to the strength behind the core development team.