That's if you believe Xanthe, he doesn't know EVERYTHING. Still mumbling about the existence of Apostles.
It may have been that way with Danica AT THE TIME (Lvl 1), but as we get to 5 WITH Memory Powers and even the Arbiter's favor; we could potentially do stuff the likes of which even Ella isn't able to do.
Think about this for a single second here, instead of hating on Xanthe and always doubting his knowledge. Why would Xanthe tell the MC he shouldn't pursue it, if it was possible? He would've encouraged it, not persuade the MC not to ruin his own power going around collecting half baked powers and weakening himself. Like, this is Xanthe, the guy who's willing to go far to test out theories and powers. This same guy is telling you, it'll ruin you and you somehow still doubt it anyway?
And after seeing what the Eye did and what it wants, you really believe it's just going to hand the MC the ability to copy powers, even if it was possible, or make monsters give the MC connections to their power? If you still do, I hope you're ready to sacrifice Ella, Alice, Michael and several others for it because that's going to likely be the bare minimum requirement on top of giving your body up.
Let it die already. If this isn't enough to dissuade you guys about this silly ass power copying stuff, I hope he implements it but permanently weakens the MC's stats in the process. We'll see how much you guys love being weak with useless amounts of even weaker powers.