You don't think a lot of heralds would be cannon fodder if Galactus employed 7000 of them? I don't recall a lot of GLs being beaten easily in a straight up fight. You also need to keep in mind that their rings stop working if they try to employ lethal force. Which makes surviving a lot harder when they're fighting someone with killer intent.
Secondly, Sinestro corps sniped a lot of them and caught them off guard. Anyone can be caught off guard. Hell, you're using it as an excuse for why Thanos was killed.