The only way I could see bitcoin working as a mainstream currency would be as a reserve currency used in international or bank to bank transactions, much like gold would have done a century ago. The protocol in its current form just does not seem to be designed for mass use, but could have many advantages for lower frequency - high value transactions like this. The bitcoin protocol seems to be more of a competitor to SWIFT than to Visa or Paypal.
You need to be more precise with your terms.
Systems such as Target2, CHAPS, Fedwire, etc., exist to allow governments and large companies to settle EUR/GBP/USD transactions in real-time, with finality, on the books of the respective currency's central bank. To that extent, the Bitcoin network can be seen as the Real-Time Gross Settlement system of the Bitcoin currency. But it's important to realise that the currency and the RTGS come as a pair. Target2 for EUR, Bitcoin network for BTC, Fedwire for USD. The concept of a "reserve currency" doesn't come in to it.
(It's also worth pointing out that SWIFT doesn't really come into this discussion - it's just a messaging network..... payments are ultimately settled by the underlying RTGS.)
But while I could imagine a bitcoin-like currency acting as a reserve currency I don't see why the organisations involved would not just start their own blockchain and impose their own restrictions on who can participate in the network. I can see a continued future for todays Bitcoins continuing as an almost unregulated grey market currency, but find it difficult to imagine them going into mainstream use in its current form.
Following the logic above, then, what do you mean by a reserve currency? Typical definitions encompass the notion that a reserve currency is one that governments hold a lot of, either as an asset or as a way of settling obligations -- and/or the notion that it is a currency in such widespread usage that major commodities are priced relative to it.
In both cases, the currency needs to be widely adopted -- so it's not obvious that a government-bootstrapped currency, with strict usage/membership rules would really take off.
Your point about the scalability of the Bitcoin network is a good one, though... I'm still not persuaded that it is appropriate for mainstream use, even if the scalability limits were removed. Just as most Brits wouldn't use CHAPS for every payment even if they were granted direct access (they would prefer the easier/cheaper/safer Faster Payments system), I expect most users of BTC in the future to do so intermediated by a bank-like entity and a collection of off-chain deferred net settlement systems. Purists might not like it of course....
