Why wouldn't this be plausible? Let's say one day China had enough of Bitcoin, and used their essentially limitless resources to gain enough hashing power at will, to block transactions or rewrite them or what have you. Entirely plausible with Bitcoin (in this case, China doesn't care about the coin reward and therefore is not a "rational attacker" as the popular game theoretic model of Bitcoin security presupposes).
Now let's look at the Stellar model in this same situation. We've got a bunch of large company nodes that are probably Gateways (for the sake of argument say JCB, Wells Fargo, Barclays, and Bank of Brazil). We've got a ton of other nodes that belong to research universities, and then we have a bunch of "non-profit" or hobbyist or whistle blower nodes. There's a nice graph topology between all of these. Then one day China comes along and decides its had enough. How does it attack the network in this case? By hacking enough organizations to take control of their nodes? Seems a bit more unlikely than it gaining 51% of hashing power on the Bitcoin network...
