It's not even the volatility directly, but the long term potential that's preventing people from spending their coins. People don't want to buy a TV with $1000 worth of Bitcoin when they strongly believe that it can be worth +$2000 a year or even months later. It would sting even more when you spent a large amount of Bitcoin back in January last year, where at current levels it means that your coins could have gained 900% in value. I'm sure that if the market would grow with just a few percent per year, the barrier to spend coins would no longer be there. I do believe that it's just a matter of time before we reach that state, and that with lightning network is a killer combination.
I was thinking more about the merchants, since they are having a more conservative approach to their finances than their customers - many of them view accepting Bitcoin as a huge risk because of it's volatility, as they can lose a lot of value if it crashes. A lot of companies that were or still are accepting Bitcoin are actually using proxies like Bitpay that immediately convert it into fiat.
As for people unwilling to spend it due to belief that it will increase in value, I think it's okay, since storing value is pretty big use case and it also counts as adoption in my opinion. Even if we for a moment imagine that Bitcoin will fail as a currency (although there's no reason to think so now), being "digital gold" is still a huge achievement.