You haven't read the Rise and Fall of Commodore book or looked up Mehdi Ali have you? That will give you all the answers you need as to why A3000+ with DSP was scrapped and A4000 was so crap when released.
Anyway, I think the end was when they sacked most of the original Amiga Los Gatos team (i.e. the ones who actually did the real work and made the Amiga 1000) in 1986 when they were working on 1024x1024 gfx with 128 colours. This showed the future direction of Amiga product line management and (lack of) R&D investment. The A500 and A2000, although extremely important in market share, were just cost reduced utilisations of the R&D invested into the A1000. Let alone taking those same short-sighted decisions even further with the A500+ and A600 into a dead-end of technilogical obsolescence.