What you say is true, but still having him die at 16 years old is an extremely wrong and depressing ending for the saga of such a great character. It would be like having Batman die in Year One. It's just wrong...
If they absolutely had to get rid of him, I would have preferred for them to keep him around a few more years and let him age a few years, and then, when he is in college, he could have an unplanned child with Mary Jane, and decide that his new responsibilities as a father wouldn't allow him to continue as Spider-Man. Combine him with some problems with his enemies targeting his family and it's time for him to fake his own death, with Fury's help, and retire.
Of course, with Bendis at the helm, aging a few years would probably take 20 years of stories
I also agree that Miles is not as well-defined as a character as Peter. Still, it's a bit unfair to compare him against one of the best two superheroes ever. Also, he's still so young... Maybe we readers can find out who he is it along with Miles himself.
Anyway, it's also true that people want to read about him because he is Spider-Man. It's a rather lazy way to make a character successful. Just kill Peter and let Miles take his mantle. The problem is that I don't see new characters really succeeding on their own, even if they are good.