That defense is a pretty handwavy answer. You can't sum up the high priority transacations(whatever that is... based on bitcoin days destroyed, I guess??) against hashpower without assigning weights to each, so what are those weights? And even then, you're simply trading one type of security, hashpower, for another, high priority coin ownership. It's a risky idea, by closing one avenue of attack, you open up another, only you're not familiar with the second. Computer programmers have the nasty habit of assuming if they don't see a flaw, there isn't one, and historically this blows up in their face a lot.
If the old network is attacked and falls because of the majority of miners attacked it, it does not mean financial suicide. Rather it's that if you don't follow whatever fork the majority of miners choose you are in danger of losing your transactions, and therefore the value of your balance. Bitcoin has always been based on that notion, "the moral majority will have the most hashing power", it's just that now the owners of that hashing power are centralized, cloaked in secrecy, and probably far from moral.