Well, there are a lot of different opinions on both sides of this issue. My own opinion is that if Bethesda allows modders to sell their mods, it will change the fabric of the community that has developed and risk undermining the sprit of sharing and cooperation. For the past 13 years, Bethesda has mandated through the EULA that all mods for its games must be purely non-commercial., so no one ever really expected to profit directly from their mods, although you could profit indirectly by building a name for yourself and using that to get a job with a game developer or starting your own company. But their was no expectation of selling your mods.
Despite the fact that there was no expectation of making money, thousands of modders from across the globe spent thousands of hours making thousands of mods, many of which are simply incredible. Requim, for instance reportedly took over 2,000 hours to create. I am sure Better Cities took many more hours than that, since it was started by Bannasplit and then he moved on to other things and let the project be taken over by others. Its now being managed by Vorians, but many people have contributed over the years.
Throughout the past 13 years of TES modding, there was a spirit of sharing and cooperation. Granted, not everyone has always wanted to share all their works all the time, but by and large it has been about sharing. I am not just talking about sharing mods with users, but of sharing resources (like texture and mesh files and other mod components) with other modders. Introducing paid mods risks shattering that atmosphere of sharing among mod makers, since people will naturally be more protective of their creations if they think there is money to be made.
Large mods like Better Cities and Requiem are just too big to be realistically made by just one person. They require a team of people working together over an extended period of time, and they require resources made by others. As long as no one is making a profit, people tend to share their time and their creations freely, which makes these large mods possible.
Under a system where people can sell these mods, I fear that you would not see any of these larger mods being created since it would be difficult to get everyone to cooperate and share, unless you ran it like a business. And as soon as you start running it like a business, then profit motive is going to take over, like it must in all businesses. At that point, instead of getting mods like Requiem that deliver an old school hardcoe roleplay experience, you are going to get the same kind of streamlined, watered down stuff that Bethesda releases in its vanilla games. Because that's where the money is.
So, that in a nutshell is why I am against paid mods. It's not out of any sense of entitlement. I personally don't think that mod users are entitled to anything, and they should be very happy for what they get. It's not because I don't want to pay for mods. I earn plenty of money in my day job to pay for mods. It's not because I don't think mods are worth paying for because some of them are priceless. But I do think there is a significant risk that introducing the ability to sell mods will shatter the modding community and make people more protective of their own creative work and less willing to share it with other modders, which in turn could lead to fewer really big and truly inspired mods and more high quality but smaller mods with a few really big ones that are aimed at the mass market instead of the much smaller market of serious roleplayers.