Unsolicited advice: I think you're right, nobody will pay you for it. Every serious programmer interested in game dev writes engines as a hobby. Just search through GitHub. It's partly because it's just tech exploration fun..

If you're serious about games you should make a game. Build an engine too, but only build what you need for the game. This will guide you in making the countless tradeoffs necessary to build an actually shipping piece of software. Otherwise, it's just a tech demo with no constraints or commercial value.

The general game engines (Unity, Unreal, etc) that are commercially valuable spent a lot of time and effort on excruciatingly boring things: supporting a lot of file formats, having very stable tools, being very accessible for beginners, etc.

I worked in the game industry for years and left it, I've made games before.

The engine I'm making is for a very specific need (initially made to help someone else make a specific game), a few people have been using it to make their games, but it's a pretty niche thing for narrative games - https://get-narrat.com/

I'm making it for fun, but I don't expect something like that to ever make money, for that I'd need some big companies to decide to sponsor it and pay me, which is super unlikely.

Don't be discouraged. You're more likely to succeed by targeting a niche.

Sounds like fun. Disregard my comment then.