Yes, I agree they could use one of the existing stablecoin but I think Facebooks wants to have something they can fully control. Or maybe existing ones don’t completely satisfy their requirements.
In the beginning I don’t think anyone is going to convert their savings to Coin, but what I feel would happen is that once people get cross-border payments in Coin, they would rather prefer to keep it as Coins instead of paying to convert it to a local currency. But this will only happen if they can use that Coin for things like paying bills, topping up mobile phones, paying for Uber etc. As for Facebook having control over someone’s funds, that’s similar to traditional banks and people in countries with less concern of privacy might not bother too much if its gonna offer them more convenience.
Maybe stablecoins as such don’t need to be centralised, but when pegged to dollar or traditional currency, like Facebook plans to, I think they need to be. The decentralised or the crypto-collateralised version you suggest would be ideal, and I hope over time Coin evolves to something like that. I’m definitely not in favour of a tech giant having control over a popular currency.