I think the best argument for one ledger is network effects. Network effects for money are very very strong. In order to enable a switch to a new money, there has to be a significant improvement over the existing money. Network effects are so strong that people continue to use fiat even though it has been inferior for decades. To me this is the main reason why Monero and other alts will not win, even if they offer some advantages (which I think is debatable), the advantages are not enough to overcome the network effects that are already behind bitcoin (which itself may not be able to beat the network effects of the dollar).
I agree. This is another factor that prevents altcoins from reaching escape velocity instead of profit taking arbitrage back to BTC.
This is why I say new markets are critical if you want your altcoin to reach critical mass.