Suppose they become the main bitcoin bank, and if they offer instant transaction confirmation already this will attract merchants and users. If there aren't any competitors, they could one day decide to charge you 10% on all transactions. And so you have a choice: either have instant transactions that cost 10% more or not have instant transactions. Now, this is a very far-fetched situation, and more than likely there will be more banks in the future offering instant transaction confirmations. Everyone wants to jump on the band wagon once they know that it's profitable, don't they. So more than likely is that we'll have more bitcoin banks popping up that offer the same basic service to merchants - instant confirmations. Now here's where it becomes a problem: as there are many banks, they compete with each other for users, so the market is fragmented (especially considering an almost nonexistent barrier of entry). For users that might not be so much of a problem as it is a benefit, but think about merchants for a bit and you realize that it's beneficial for merchants to work with as many banks as possible that would allow them to receive instant payments. It would be a huge pain in the ass to review each bank, not to mention that it's also a risk to rely on this instant confirmation before the transaction is actually confirmed by the network. Now lets forget about these issues for a moment and focus on the experience of a user that tries to pay with bitcoin: the user clicks a "pay with btc" button on the website and is shown a page with a list of banks to choose from, now the user has to search through the hundreds of banks on that page just to fucking make a payment. Compare this with paypal or google-checkout and it's obvious that the user will prefer other payment options to bitcoin.