It saddens me to see this happen, as it is not just Sonar, but also Rapture Pro (Dimension Pro) and Z3ta+ that will cease development. I do not think anyone at Cakewalk saw this coming, as they were still celebrating their 30th anniversary and were really improving in the last 2 years and especially last few months. I hope another party steps in and acquires them, as nothing good can come from staying at Gibson... they have called Opcode before and Deckadance (under the Stanton brand).
SONAR was my first DAW when I was like 15. I eventually "graduated" to Pro Tools when I was running real recording sessions later in my 20s, but Cakewalk absolutely opened the door for me. Sad to see them go, especially since I have a few friends in Boston who work there.