You can definitely use HDRI in Terragen, and they work well.
And you are right, skies were a hot ticket, which means it substantially will impact TG sales, as that is what people get TG for. Besides us people on the forum, people get it, ask a couple questions about atmospheres, and are gone, cause they got the use they needed.
From a business stand point, it's a absolutely terrible decision which will ultimately run down Terragen which is already aging too fast and becoming antiquated.
And TGs framework is very flexible, there is no reason atmospherics couldn't be GPU accelerated within TG and let CPU chug at terrain or objects. In fact this would speed up Terragen significantly as atmospherics are the main hindering point (when not working with ridiculous lateral stuff).
Imo a dedicated HDRI app won't be as great in the end. They don't exist for the simple reason a lot of HDRI in film and such contain unique elements. Think Star Wars, Marvel, etc, etc. Whether planets, effects, or what not. Even HDRIs done in TG recently for film have elements that seem they won't be possible in Terragen Sky without Terragen to handle it.
I am also worried this is just the end of Terragen. Things have been to slow, and to hear about Terragen Canvas years ago, and then now this, while seeing nothing substantial done to TG, it's really shocking.
