I am afraid that if everybody is holding bitcoins, transactions will decrease and there won't be a bitcoin economy
I think your wrong, if everyone holds bitcoins then less bitcoins will trade in the market and buyers increase then price will automatically increase.
And when its price hit US$300 people will sell some of their bitcoin, then the price will be decreased again. There is just a little chance for its price will reach atleast US$500 for next 2 years imo.
2 years in the future will be more and more users bitcoin, and bitcoin only 21 million, so the largest possible bitcoin prices surely will further increase