These weapons are meant as end-game rewards for players capable of clearing the content, just like the crowns and the dimension weapons.
Access to these weapons and their enchantments is an added incentive for players to visit D1 & D2 and increase their reputation. More players farming in D2 -> more supply of runes on the market.
Currently, runes are very expensive and they're not going to get cheaper if no one who wants to use those runes to make TG weapons also wants to farm for them in D2.
So the big downsides you didn't think about are 1) the effect on player activity in D1 & D2, and 2) the effect on the market.
Don't remove incentives for players to visit non-instanced locations.