The future might bring us to a point where coins begin life with a PoW approach and then switch over to PoS after theyve all been mined. Or they might release all possible coins immediately upon going live and stay PoS all the way.
"The future"? Both variants are pretty much standard since 2013 or so 
Lets not get too complacent. I remember a time when myspace was the dominant social platform, I remember a time when VHS was the preferred media format, I remember a time when the simpsons was still a good show.
Well, you're of course right, but there are also other examples:- Google - dominant in its category since the early 2000s (~15-20 years)
- Microsoft/Windows - dominant since the early 1990s at least (~30 years)
- Coca-Cola - dominant since the 1950s ? (~70 years)
Bitcoin is dominant, basically, since 2009. Almost 10 years. Myspace was dominant only for 5 years, so Bitcoin is already doing better.
The reason why I don't see much competition for BTC, in the short term, is however that almost all other coins are following a centralized or semi-centralized paradigm which won't attract most people attracted genuinely by Bitcoin. (I heard that Dash is about to declare insolvency, for example. That won't happen to Bitcoin
)(And since I heard about the 0,02 USD/kWh solar plants in Mexico I suspect that PoW can be sustainable. It may be not ideal, though.)