It turns out that the 5e rendition of Spelljammer will discard crystal spheres and replace the phlogiston with the astral sea. I'm not surprised and this is similar to my own past musings on how to streamline the convoluted nature of space travel in D&D. Initially I was skeptical of this change because astral space has such different planar properties, like timelessness, but ultimately the most I can do is make minor quibbles. I think this change was a net improvement, even if it wasn't perfect. The naval aesthetics of wildspace and astral travel are already a big reason to condense them. It even makes sense in an etymological sense: astral literally means "of, connected with, or resembling the stars." Why wouldn't stars be a part of the astral plane?