It's true that 'not your key, not your coin', and that there are several wallets and services available to cover yourself.
There may be a point at which any cryptocurrency (whether Bitcoin or another) is held on exchanges/wallets run by small/new companies.
The small players have limited resources. How do they ensure they are sufficiently protected? Why don't there be hacks every other day?
Helding coin in a hardware wallet is nearly impossible to be hack since no one can access your coins unless you lost the device. For helding the coins in exchange is like gamble for me, i can deposit for a short time but i will never stack my coins there unless i have to trade it immediately.