I'm not an economist, but economists are not good at speculating either, looking at the results: As a a team, economists failed to preview every single financial crisis. Only after the shit hits the fan, media goes back to the few who were able to predict the crisis. There are always solitary birds singing out of the chorus. The chorus never get it right though.
To your statements now:
1) That's true, but doesn't mean anything if you put it out of context and the context might very well change in a couple of years. Crypto-currencies are evolving now, new ones are created trying to tackle bitcoin's flaws. Now seems unlikely but in the future some might be more successful than bitcoin. It's a new sector.
2) There no central governing body, but an investor with a big amount of bitcoin (say 2% of the total) might easily drive the price up, down or side-ways. It's like a financial (assets) market really, a little bit of miss-information and a massive dump of bitcoins will turn the price down at once.
It doesn't make any sense for a merchant to accept bitcoin if:
1) He is not the guy holding a huge bitcoin portofolio or plants too. Say Starbucks as a corporation starts accepting bitcoin today, in 6 month does a massive investment in the bitcoin market. If the investment is big enough the price will go up (supposedly).
2) If they have some inside information which allows them to predict with an acceptable risk the future price of bitcoin.
Other than that - and I'd go for that theory - it doesn't make sense. It's just a marketing trick. They are not expecting big amounts, just very small amounts because as you said bitcoiners are basically short or long term hoarders. But I'm sure they are ready to absorb any bitcoin drop up to a point, but not further. So I'm positive they wouldn't accept 20% of their turn-over being on bitcoin (if they are not feeling suicidal).