I wish I could concentrate on ArtWorks but the amount of money I can charge per year is limited, so that effectively limits the amount of time I can afford to spend on developing it. At the moment, I am working on many projects at the same time and I am starting more soon. Apart from ArtWorks (and the renderer) I am working on EasiWriter/TechWriter, GhostScript/GView, a big non-RISC OS project and soon I might start porting Gimp-Print 5 and there will be yet another new big RISC OS project. Unfortunately, that leaves less and less time for each individual project.
I was not actually looking for other income streams as you suggested, I have already got more than I can fully handle. Having transparency in the renderer was simply one of the biggest wishes of many customers and I am very happy that I eventually managed to do it. I did not do it in order to gain income, I could have got that more easily by developing ArtWorks itself, but I did it because it was such an important thing to have. However, having spent the time upgrading the renderer rather than adding a new ArtWorks feature or developing a new product, I now need to gain some income, which is why I am selling the upgrade. ArtWorks has gained enormously by the new renderer but that still does not mean that I will sell a single extra copy of ArtWorks if I make the renderer available for free, so there is no way to recover the development cost from new sales.