It is intended to be an upgrade, so no plussing level is transfered. This way everyone can farm the normal artefacts through trashmobs while not being forced to clear the entire instance. But the refined artefacts require loot from the final boss and do require full clears. So if a group of players is not able to clear the entire instance yet, they can still achieve the normal artefacts. And stronger groups can go for the refined ones additionally. Everyone wins.
So your assumption about weaker groups is wrong. If weaker group is not able to do full instance they will not unlock the artifacts and there is no point for them to farm anything.
I think there is a big misunderstanding: The loot that has been removed as a hotfix to remove the possibility to skip the content is returned to the monsters, so you will still be able to farm the normal artefact. For the refined one though, you will need to finish the instance. The content that is in the game right now will be retained, it's farmability even reverted to when it was first released, but it's improved counterpart will be locked behind the endboss.
Kalin is supposed to be hard, something to give the playerbase something to strive for. Weaker groups are not supposed to be able to clear the instance. These groups should otherwise try to find ways to improve.