The modding will no longer be able to truly edit the engine itself through some reverse engineering and be forced to utilize the APl/scripting framework. Third-party tools will be locked out. Obviously this has happened already on the console platform. There's still the PC platform but that could be locked down further as well.
Think about the time and fostered talent that it took to make some of the communities amazing tools. For example script extenders for elder scrolls series. As mods are now centralized in official 'the store' the community grow around which will never allow mods like the script extender for developers to make advanced innovative mods. Even if other modding communities like the Nexus allow for that It's going to continue to fragment the community and the talent which is foster within the community. Then you throw paid mods into the picture... Thus begins the death of the open source pillar in modding.