You got your live feeding routine to near perfection which is huge and happens very, very seldom from what we can see going around. It is very hard to do and you accomplished it, so congratulations are in order! So the danger here is in generalizing, ..... and absolute, all-sweeping statements are nearly never true anyway. I don't think you'd disagree that for 90%-99% of hobbyists, feeding live is risky but, as you and MonsterMinis and Co prove it, it can be done right in terms of the disease transference. And that's that.
As for the cruelty, yes tis' such a grey area. Yes, we accept the "cruelty" of keeping animals captive, of line breeding, selection breeding, cross-breeding, etc. for the higher purpose of learning about the animals and having them help us (e.g., dogs). Clearly there are more humane and less humane ways to keep a fish or any other animal, wild or domesticated. People know this line much, much better when it comes to mammals and maybe birds, but 99.999% are clueless when it comes to fish because of the communication barrier, so to speak.
Anyway, yes, the captivity (of predators in our discussion case) is acceptable by all/most people (except maybe PETA people) for the sake of the higher purpose. Let's not mix it with the cruelty to the prey. Yes, keeping fish captive is not ideal (ideal would be if we could swim like the fish with them in the wild but let's stay reasonable). But adding "avoidable" cruelty to the live prey offered to them makes things worse.
I am sure it can be argued the other way around, that live feedings lessen the "burden/cruelty" of a large predator being held captive and/or beneficial to their health, vigor, vitality, psychology, etc. And this is why I cited the zoos and public aquaria and animal-based amusement parks, etc. as those people are PROFESSIONAL wildlife keepers and have animal psychologists and scientists on staff and do research including on this topic and read cutting-edge scientific papers on this topic and experiment with their subjects (like the well publicized orca and dolphin research). It could be wise to humble ourselves before those with better knowledge and follow their example, even if we don't understand all the whys and what's for now. Neither do they but at least, for sure, they know far, far more!