I understand this viewpoint but the NFL has a compensatory draft pick program and it’s quite possible they could’ve picked up an extra 3rd, 4th, or 5th draft pick had he left after next season. As it stands now, they may not actually get any additional picks from trading Hockenson.
I will add the caveat that I expect the Lions to be in serious FA acquisition mode after 2023 and it’s quite possible they will acquire more FA than they lose and wouldn’t qualify for any additional picks.