In actual fact, the Amiga has
less claim to the 'A' prefix than the Acorn Archimedes, which is actually marketed with the A.
Look at an A3000 (Archimedes) and you'll see it says "A3000". Look at an Amiga 3000 and it says "Amiga 3000".
Admittedly, I wish my A3000
was an Amiga 3000

However, be assured that no one computer has the right to the 'A' prefix, and if one does, it's the Acorn Archimedes.
That said, I still think the 'X' prefix is better, and if we do use the 'A', we should avoid numbers already used by the Amiga.