You are right that Minarchy tends to lead to government growth and bloat, though that was under the fiat paradigm. With Bitcoin, statist growth will be limited.

The assumption that small, decentralised security groups will inevitably consolidate into a state-like entity presumes that people will allow such monopolisation to occur. However, if individuals and communities recognise the inherent illegitimacy of aggression... even when labeled as "government"... they would reject any group attempting to centralise power by force. As long as you believe in authority, this is a very difficult concept to wrap your head around. I recommend you read the book, Most Dangerous Superstition, by Larken Rose.