I think, from Bethesda's point of view, it would better to monetize TES 5 modding partially and then finally replace modding in TES 6 with an ingame store. People will be angry, but this doesn't hurt the sales much. The largest portion of Skyrim's sales was made up by the PS3 and XBOX360 audience. Until October 2016, above 30 million units of Skyrim have been sold. These people (console players) don't care about modding, they usually never even heart something about modding. Console players would also pay for microtransactions. They are glad to receive any additional content. You can always do some DLL injection with memory manipulation to change some basic stuff (or more extended - after painful months of reversing a game engine, see OBSE or SKSE), it is usually a tedious task to reverse engineer meaningful components of the game logic. Advanced modding in Skyrim is possible, because the engine's core components have been reversed over the course of many years and Bethesda also provided a powerful modding framework for each game. There are games like Rise of the Tomb Raider, which doesn't even allow simple texture replacement (as of now, even after 2 years of work on it, only very basic things can be changed like the color of clothes). If the game file formats are completely unknown (unlike in for example ES or GTA games), reverse engineering is a painful task.