"Trust me, I'm not letting you go without a fight." That fight will be drowning him in Istredd's mind and memories and feelings and trying to give him something to hold onto and remember. He doesn't mean a physical fight because obviously that would be hilariously pointless. But emotional warfare he can partake in. He may have to accept if Lucifer gets lost to him, but he won't lie still and let it happen.

"We'll all hold onto you, Lucifer, as long as we can." Wanda, him, Wilhelm, Kell, Kyle, the people who have come to care for Lucifer, and he's allowed in his life. And this is after one year, there may be more as time goes on, and enough people to dig in their nails.

Istredd kisses his forehead gently. Let me worry about your beautiful mind a decade from now then.