Bitcoin is sometimes vulnerable like getting stolen from phishing sites, hacking and scamming other people is a form of damage to bitcoin.
It's not just "sometimes vulnerable" bitcoin is always vulnerable to attacks all the time, the safety of bitcoin actually depends on the holder, if you leak your private key out to scammers your bitcoin will definitely get stolen, so it's just like a "wise holder = a safe bitcoin, a careless holder = an unsafe bitcoin".
Is there any chance that bitcoin can have an insurance from its vulnerabilities in the future?
As you said already, "Bitcoin was created as decentralized digital currency" to eliminate the middle man, so why do you need a middle man now?, the only insurance you can get is keeping your private keys or seeds safe, nothing else, if they are kept safe you won't need a middle man for insurance.