So I rather see it like some common "bitcoin paypal API", that would allow a merchant to hook to a new instant payment provider with no much effort.
You make a good point, and I certainly don't want to see the necessity for bail-out funds etc when one node in the settlement network goes under. I like your alternative, and it's probably more realistic, but I'm afraid it would create a much smaller ecosystem, perhaps tending towards a duopoly or worse, like one "Visa" and one "Mastercard". I want a rich ecosystem of actors competing against each other on fees, security, features etc.
That is why you want each of these paypals to provide you with different kind of services - that is going to make the difference.
Not everyone needs a Tor level anonymity and a highly secured con mixers - but some do and they will be willing to pay for it.
Such kind of providers might not be accepted by all kind of merchants, but can still use the same API.
At the other hand, if you provide a bitcoin paypal with your photo ID and a credit history - they may like to build their business model not as much on transaction fees, as maybe on crediting your payments, assuming you'd want to borrow some coins from them at the interest.
BTW, both of the above mentioned business models are evil for some people.
The first one feeds on a money laundering - the second on usury...
But none of them is evil for me, especially when people have the choice between them.
Not to mention that there would always be business models in between.
Still there is no reason why the API could not be common for all of them.