I basically like your arguments and agree with them. I agree that high volatility is a non-issue in itself. Nevertheless you are missing one important aspect: transaction fees. They are low but whenever the volatility gets high, they get insanely high. And I think that the biggest issue currently is high transaction fees in times of large volatility and it is in fact a blocker for bitcoin being used as a currency.
Good point, but I think the Lightening Network should eventually resolve the fee problem, and as I understand it fees are already a lot lower than they were during the last feeding frenzy. Eventually SegWit will become the dominant address format, helping to keep fees under control. But also, if Bitcoin fees do go sky high, more and more people will be willing to use Ether or Bitcoin Cash or Litecoin instead. Excessive Bitcoin fees will eventually go away completely because fewer and fewer people will be willing to pay them and will know how not to.
I am sceptical here. Lightning Network is just an off-chain service (a third party service) trying to address the issue of high fees but I wouldn't be surprised to see it eventually abusing the faith people have in it and introducing fees of their own. I have seen it again and again with innovative online banks having a free (and free forever, as the ads would go) service only to introduce fees when they attracted a satisfactory user base.
What we need is an on-chain solution for high fees, SegWit was a part of a puzzle here, but not a whole answer, obviously.