I agree with you, although I can understand why from a dev resources perspective they have been focusing on lumen more. Performance is massively better compared to where it started at, but image stability is still enough of an issue to warrant better baking.
It’s a good question, although I will say that combining baked lighting with DF lighting has been a very successful in the past (Stray made great use of it). I wonder if you could have the SDF look up radiance from a volumetric lightmap, which would give low-res but still color-accurate GI. Expense is a great question though.
Slightly off topic too:
Summary
All that said, if you want a really good implementation of RT reflections and baked lighting, look at the Callisto Protocol. Their SIGGRAPH 2023 talk (SIGGRAPH 2023 Advances in Real-Time Rendering in Games course). Since most games are going dynamic it makes sense that it’s not a priority for Epic, but there are still situations where dynamic GI isn’t performant.
That reflections solution in Mafia 3 is really interesting as well. I think I saw God of War use something like that too (they called it Ray-traced cubemaps or something else), which overall gave really high-quality especially for non-mirror surfaces. It’s a brilliant idea and one that’s perfect for pretty high-quality dynamic reflections, but I think with how quickly RT performance is improving given new innovations, it probably won’t be implemented going forward. Also, the mirror issue is really fascinating. The temporal accumulation for the SSR broke somehow, and samples got reprojcted into the cubemap? It was an interesting bug. Mafia 3 still had some of the best mirrors I saw outside of an RT game.