I conjecture that efforts based on a freely accessible Bitcoin network will have a very low barrier to entry and can arrange an endless array of unique incentives so I very much doubt that they would fall victim to the natural monopoly effect.
You haven't explained why the tendency for a few dominant networks to arise, which we see in Visa/MC, would not exist with payment processors handling BTC-credit. BTC-credit is not BTC. BTC's decentralization doesn't make credit held by centralized BTC-banks less centralized. The handling of credit happens through traditional off-the-chain channels, where BTC's decentralization has no benefit.
Again with all due respect, it seems like the 'offchain' crew is making assumptions that are not supported by any real substance. The safer assumption, in my opinion, is that centralized parties handling BTC-credit will act like banks that handle other types of credit now, and will be vulnerable to the same type of pressure.
And as I say, with all due respect, you seem to also. I reject (fairly vociferously) that there would be a mechanism to promote significant centralization while you assume it would be the case.
I believe there would be a mechanism that promotes centralization, because there is a mechanism in traditional payment markets that promotes centralization, and traditional payment processors handle fiat-credit, which has almost exactly the same properties as BTC-credit.
You haven't explained why this mechanism, which we see in fiat payment processing, wouldn't be seen in BTC-credit payment processing, even though they both use the same type of trust-based credit relationships.
There is very little to attack if there's a 1 MB block size limit. BTC won't be a major player. It'll be centralized banks that handle credit redeemable in BTC that dominate the BTC economy, if indeed BTC becomes widely accepted, which I don't believe it will if transaction fees need to reach $20 to handle global scale traffic.
I'll grant that in the unlikely event that bitcoin succeeded under a 1 MB block cap, and this kind of economy came to be, at least the currency wouldn't be inflated by governments, but everything else will be like the modern financial system, as far as I can see.
Bitcoin is not THAT useful as a technology. Never was, never will be. If we artificially cap the transactions to some level (as opposed to letting the market decide) it will certainly make bitcoin even less useful.
That's exactly how I see it.