Do payment processors who convert bitcoin to dollars (like Coinbase and Bitpay) devalue bitcoin?
For every Bitcoin they sell, someone else had to obtain that Bitcoin. So they increase the demand for Bitcoins by at least as much as they increase the supply.

In addition, they increase the utility of Bitcoins, which increases their value. All other things being equal, a currency you can spend at more merchants is more desirable than one you can spend at fewer merchants.