I can say that they "leveraged" it (by saying hey Bethesda, would you like free profits that you wouldn't have otherwise been getting? Just let us be the official mod distributor), but that is not sufficient proof that it's detrimental to the consumer.

How would you view it differently if the situation was completely identical, but Bethesda running the mod shop instead of Valve (not likely in their case, but entirely plausible if it was Blizzard instead, for instance)? Is that suddenly a "monopoly" on mods for that one game?

If you don't like the business practices of Bethesda you're free to not buy their games, obviously. But if you don't like the business practices of Steam, you're free to not buy games that require Steam, too. You will lose the ability to purchase things that are exclusive to them, just like you would lose any PS4 exclusives if you stopped buying Sony products. You certainly won't lose access to all PC games, or any console games; just a certain subset, like with any other vendor.