Firstly this game’s legendaries are completly different to the concept on most games. Rares and legendaries have the same base random statistic rolls. The difference being legendaries can roll with a power attached and this can be higher than the generic ones that you get from the dungeons.
Not sure if they are going to add truly Unique items with fixed stats or not, however the only reason to reduce legendary drop rates is to reduce the difficulty in getting the perfect legendary power, as you will just turn your perfect stat rares into a legendary anyway.
Personally I am ok with them not having true unique items. I rather enjoy this psuedo crafting system where you find a good base rare/legendary and then add your special power too it. It is a refreshing take on itemisation.
They do need to get the balance right though so that you don’t have 5 of your special power saved up before you find the right item to put it on, with the dungeon enchants being the middle ground it is not a bad idea to have the legendary items have a lower drop rate.
This means that finding the max roll power will be something special, and you just roll with your generic power until such a time, this allows most character builds to be viable throughout and gives the min maxers something to work towards.