I think Day misses the idea of soloing and improvisation in his article. First, the difference in ending a concerto piece and a jazz solo, which has been stated by others and Day himself is that an ending of a concerto piece actually concludes the song. However, the ending of a jazz solo is just a conclusion of a section of the song. I view jazz songs as conversations amongst the musicians performing the piece and the audience. Whatever the subject matter is and the structural components of the song, it is the musicians job to discuss through their instruments the tune, society, and emotion. I believe musicians speak with one another on stage and their solos are their opportunities to have to the floor, speaking through their instruments, playing whatever comes to mind. There might be a call and response with others and again this an example of musicians communicating with one another.
I do not agree with Day when he says that, "the end of the solo is, by and large, no particular concern for the improvising artist". I think that it is the exact opposite case. In speaking, we focus a lot on the first and last sentences that come out of our mouths. We want to capture the attention of audience with the first sentence and also we want to conclude with something that will have a lasting affect on the audience. SImilar in soloing, we want to capture the ears of the audience and also we want to end with something that makes the listener nod in agreement. It is the musician's job to decide what that musical phrase will be and there is no telling if their ending is successful or not.
I guess what I am saying is that, a solo is a small component of the song, it is the chance for the trumpet or saxophone or whatever the instrument is to have a say in the conversation or story. The ending of the solo is the musicians time to direct the audience's attention back to the overall conversation.