is out of their control since UE4 does not allow plugins to interface with the renderer, which is why VXGI etc. are on separate branches of the engine, rather than being available as plugins. Berating the company that makes for UE4’s (currently) limited plugin support is pretty unfair.
If you think it’s “just a sky”, then you don’t understand the plugin. Epic can afford to make their engine free because of the 5% revenue cut and marketplace sales. Simul don’t have either of those. If you think it’s a rip-off, go make your own solution - I can guarantee it will cost you more than $15 a month.