There is nothing inherent to Bitcoin that prevents an exchange from being operated responsibly and covered/insured just as well as traditional financial institutions.
> Block size can't be increased because bitcoin is decentralized.
It is not trivial, but it can be increased. What you don't understand is that all the merchants all the institutions all the individuals utilizing Bitcoins rely on its proper operation to benefit from it. If they more or less all agree that raising the block size is the obvious way to scale up, then they will agree to raise it.
> Bitcoin can not support more than a couple million users.
False. If the block size is increased, it can scale: https://en.bitcoin.it/wiki/Scalability
I invested $0 in Bitcoin. But a coworker gave me 500 BTC for free in 2010 (I never sold any).