I have wondered how the future of these two programs will pan out. I think RawTherapee is a great program and I have used it since version 2.1.
However since trying ART, it just seems to have quite a few features (masking, tone equaliser, saves snapshots, inspector) which makes it really nice to use, and as long as it remains supported, I will continue to use it as my main software.
If somebody wants an open source editor that produces good images with little effort, but still allows a lot of flexibility and creativity, I couldn’t think of a better program. For somebody moving from Lightroom, it would work pretty well - your image starts looking very similar to your OOC jpeg (the average person who has done no research will assume that the OOC jpeg is the ‘correct’ starting point, and anything different is broken), then you move a few obvious sliders and they behave how you would expect.
However, looking at its download numbers, the number of users must still be pretty small. It isn’t that easy to find by most people - if you do a search for free raw software, most will bring up darktable or RawTherapee.
I haven’t done much to promote ART, because I am not sure whether Alberto necessarily wants a lot of users. If lots of people start wanting a free alternative to Lightroom, he is likely to be bombarded with questions from people who may have done little research for themselves, or just requests to make it more like Lightroom.