I didnt know it has to be 10m :/ well i still hope they somehow improve this...
I initially thought so myself but, after further thought, I've come think things are OK as they are. The way light, both ambient and artificial, penetrates walls and small amounts of dirt is actually very useful for every purpose except creating complete darkness. For instance, you can hide big, ugly floodlights by burying them just below the ground or by putting them in some peep-free void space of a building, and they still do their job. And also, you can exploit this property to make cool light and shadow effects in a dark ride. In fact, most of the spooky lighting in well-made dark rides relies on this property.
So all in all, I think having to bury dark rides at least 10m deep is a small price to pay for the many benefits of how things work now. 10m isn't really that far, just 2.5 stories of standard 4m-tall wall pieces. This is a pretty easy limit to work with.
Thus, rather than change how light works, I'd suggest changing how building parts (and probably art shapes, too) work. Add a toggle to them that you can set to make them impermeable to light. That way, we'd still have all the benefits of how things are now, but could optionally make specific buildings dark inside without having to bury them.