Some time ago, I bought an asset pack which (part of) license says:
These projects shall be defined as without limitation a User’s movie or video or game or app or product that uses Assets or character model(s) that you have incorporated into your project in a way that the Assets and/or character model(s) cannot be extracted to allow further distribution, reproduction, modification or use of the Assets outside of your projects.
The engine already packs the assets in a way that it’s not a simple task to extract them out, but it’s not possible to avoid entirely–all games can get assets extracted if someone cares enough to get them out. On PC at a very basic level you can capture the data that’s currently loaded on the graphics card and that circumvents anything that you can put into your game to prevent it. Even console games can get their assets extracted out. Something like UE4 is popular enough to get targeted for people to figure out how to get assets out of UE4 games.
Another potential monetization model hides in licensing by allowing extraction of assets. Imagine, in-app purchases in which inspiring Game Devs could purchase assets. This concept came to me as a I was developing TheGameDevStore.VR. In which the original goal was to develop a 3D Asset Store with a Game in it. I later realized, I could build a Game with a VR Asset Store in it. So instead of worrying about preventing extraction of assets, slap on a in-game Store to sell them.