Also, let's be clear. It's not just the damage that makes Spirit Shroud better. Hex is stuck on one target until it dies and costs a Bonus Action to reapply to a new target. Spirit Shroud just works on every melee attack, with no target limits or action costs. The means no problem if you have to swap targets, no cap if you're cleaving through multiple minions in one turn, and no opportunity cost if you have another use for Bonus Actions like Chain Pact or a magic item.
This imbalance was hidden by Blade Pact being pretty weak and needing to be propped up by super strong supplements like Spirit Shroud and Hexblade. Now that Blade Pact is being brought up to a reasonable state, combining it with those old assists results in an overpowered state. That's why new editions usually wipe the slate clean of old material. You can't fix anything if you have to allow for all the old patch jobs sticking around.