This is a great post, esp. your point about those that hold the private key(s) vs. those that don't.

A change that's happening quickly now is the intro of exchanges that are decentralized (non- 3rd-party) and becoming fast competitors.

Here's three links that share really great information on how things are evolving:
• https://www.cryptocompare.com/exchanges/guides/what-is-a-decentralized-exchange/
• https://bitshares.org
• https://www.coindesk.com/solving-liquidity-challenge-decentralized-exchanges/

One last point: Coinbase owns an exchange alternative that requires more knowledge, but results in lower or no fees:
• https://www.gdax.com

Thanks -- hope this helps!