I think the main vulnerability of Bitcoin is a new release messing things up in a way that it will completely destroy people’s faith in the protocol.
By this I mean something like what happened in 2013 with the BerkeleyDB to LevelDB update. That could have ended a lot worse.
If for each release, there is a 0,1% chance of it messing things up, then given enough time some release WILL eventually mess things up.
I wonder if a safer approach would be to first test the new releases privately, in some selected nodes (and miners), that are constatly communicating with each other. I the software in incompatible in some way, they can swiftly coordinate to switch back to the old software. This is much harder to do when everyone has installed the new software because there is no way to communicate with all nodes.
(I know the software is heavily tested in Testnet beforehand, and that is a good thing, but it is not enough, like the 2013 situation showed)
