If Bitcoin were to be used as an alternative, industry-specific banking platform, what would the effect of all 21M bitcoins in circulation be on the dollar/bitcoin exchange rate? Or, would it be better to create some kind of new digital currency specific to that industry while simultaneously creating an exchange for that currency?
It is notoriously hard to establish a digital currency derived from Bitcoin that focuses on a specific niche. This has for example been attempted with country specific currencies (e.g. Auroracoin in Iceland, MazaCoin for the Lakota nation and SpainCoin in Spain, see national altcoins), and industry specific coins (e.g. MaryJanecoin, Titcoin, and PotCoin).
The main issue is that Bitcoin and other more popular altcoins have a much better utility due to broader acceptance, and the newly created coins often immediately get sold for those after an initial hype.
Specifically regarding Banking, banks actually have very different needs than what Bitcoin provides: Their environment is more one of cooperation with known particpants, where a central database would be more of a communication and audit tool rather than one to achieve consensus. This has both allowed Ripple to successfully pursue banking clients with their IOU based network, and has led to a score of banks founding the R3CEV, whose product naturally diverged significantly from Bitcoin's blockchain.
I could see how the banking industry would attempt to resist the adoption of an open blockchain by favoring such a permissioned blockchain. The latter are naturally much more efficient as the decentralization comes at the cost of inefficiency. Unless an open blockchain overwhelms other forms of money, I would expect both to survive.