Commodore should really have considered a SID + PAULA in the Amiga. It would have been the coolest music machine around!
Yeah, but in 1985 people wanted to move on from synthesized sounds to sampled sounds. When people first heard the sampled sounds of the Amiga they (most) wanted to ditch their C64s for this new amazing computer.
The SID wasn't considered cool in 1985 the way it is now. In 1985 even musicians were ditching their analogue synthezisers to "upgrade" to digital synths and samplers. Nowadays the analogue synths are back in vogue and everyone wants one.
It's only in retrospect that we now think the SID (and analogue synths) are cool again.
So at the time Commodore would have considered including the SID in the Amiga a step backwards (but yes it would have been cool by today's retrocomputing standards).