Using as a gas in an exchange doesn't necessary have large impact on the price of the coin.
The Stellar has a fixed transaction fees in lumen and they'll keep rotating in the network.
There will be no decrease in amount of lumen (unlike other exchange coins) and no sharing of dividend (like KCS).
And there are only less than 20% of lumen in supply which would increase in time.
I believe Stellar actually collect the fees from transactions (for anti-ddos) and periodically redistribute them to holders or whoever the voters have voted to receive their % share of XLM. It is said that the small inflation is necessary and calculate to prevent "coin death" over time, due to people losing their private keys, or deaths, and etc. gradually reducing the liquidity of the XLM currency within the Stellar Network.