There is absolutely no problem with a mod creator charging for his own time and effort put into fixing a poor UI, because he is not affiliated with Bethesda. There is however a problem with a game that is released with a poor UI.
If you buy a car and it comes with dangerously cheap tyres, the parts shop on the corner shouldn't give away a new set of good rubber for free either because it isn't their fault that the car manufacturer cut corners. If the parts shop is well aware that your model comes with unsafe tyres and asks desperate buyers a fortune for a new set, well, looks like your car came with a hidden cost and the manufacturer scammed you. You'll probably buy your next car elsewhere.
In other words, Bethesda will no longer be able to get away with releasing a game with a poor UI (or major bugs, or mentally handicapped NPCs, or awful perks, or the worst implementation of magic since D&D) because the argument that "the community will fix the whole game for free anyway" no longer applies.
If this means Fallout 4 is unplayable until you spend €20 on mods, well, there are always other games.
but I guess you're following in the footsteps of Midas Magic.
The creator of Midas Magic is a complete idiot. You can't ask money for a spell pack, and certainly not that much money, and you definitely can't put nag screens into the free version. He was greedy, felt entitled to far too much, and got exactly what he deserved.
Realistically, none of my mods can be sold for money. Very, very few mods can be sold for money. (Sure, people are going to try and sell a single armor for €5. Good luck finding any buyers.) The only mods that can be sold for money are DLC sized quest mods.
And honestly, any incentive to encourage people to make more awesome worldspaces with voiced NPCs and intricate questlines is a good thing.
take a stance that will essentially ruin modding for the younger audience
No. The existence of a "price" field on the upload form will not ruin anything, just like guns don't kill people and drugs don't take themselves.
If anything will ruin modding for the younger audience, it will be the community itself getting greedy and asking too much for too little.
It is not up to Bethesda or Valve to prevent the community from tearing itself apart and descending into all out war over a few pennies. If we are too immature to handle big people money, we don't deserve to be protected from ourselves. Deshi deshi basara basara!