It is now unlikely Bitcoin will ever receive "mainstream adoption"*. I used to think it was probable, but then there was the blocksize debate, then the DAO hack, and then the Bitfinex hack, and finally the NSA hack.
Bitcoin development is not geared toward achieving mainstream adoption and now the brand is not associated with anything that mainstream people are interested in. It's not even going to be mainstream among hackers and sex workers, and if it's not popular with those groups what chance does it have?
Why should I buy and hold Bitcoin using an ETF mechanism? Bitcoin wont scale, the brand is currently damaged beyond repair, so why would I buy it?
Personally, ive never understood the appeal of widespread adoption in the first place, at least on the merchant side. SO many of my BTC friends are convinced that its somehow crucial to the platform that they can buy their morning coffee or a pack of smokes with btc.
That said, among the unbanked countries and countries where banking is very risky, , its still (and almost certainly will always be) the coin o the realm.