If these legendaries are "shit," as you say, then why on earth are you powering them up? Why keep any of them? If you believe they're bad, then don't spend your resources on them. If you truly hoped you didn't catch the one you encountered today, you could have fled from it. Instead, you threw at it, so clearly you did want to catch it. Also, there's a perfect solution to your "concern" of not wanting to encounter one that has better IVs than your current one. Stop raiding them. You're guaranteed to never encounter a better one than the one you already have that way.
You've been writing a lot of posts lately about how dumb Niantic is, how awful the movesets are for some of the new Mons like Giratina, and how the game is heading in a bad direction. And yet here you are, doing Giratina raids, investing in a "bad" legendary, and supporting the game you're constantly bashing. You're about as contradictory as they come.