Personally, it's a one hand and another hand sort of thing, with me.
I mean, on one hand, I'd have no anger or disgust toward a good manga artist who loved Tezuka's work continuing the story, provided he had acquired the rights to do so. I might even enjoy it, if it didn't take too many creative liberties ( like, oh Astro was given an adult robot frame, and has a robot wife and now a robot son----actually, that would be awesome to read, nevermind)
On the other hand, I would never consider it official Astro Boy canon. Osamu Tezuka more or less finished the storyline and I'm satisfied with that.