What's a "crucial change"? Anyone can change the source code and its protocol rules, but to change the consensus of the network requires convincing other people to run your software. Most crucial changes (consensus changes, like 21 million hard cap), if adopted, would practically result into a network split, where a few nodes run rules A, and the rest rules B.
Sounds like bitcoin is kinda centralized to a handfull of maintainers who hold the keys. What happens if they all die in a car crash or become dictators?
If they die, project moves into different hands. Practically, this could be done by other Bitcoin Core developers forking and maintaining Bitcoin Core. If the present maintainers become dictators, then... Well, they have to achieve the impossibility of enforcing consensus changes on an open, global, permissionless network. Pretty tough job for a dictator.