There is poor management and then there is Commodore's very poor management:
- They didn't keep up with the production processes of their custom chips and discovered at the last minute that everybody else's custom chips were able to clock faster than theirs.
- When they found out that it was going to be expensive to produce custom chips at a competitive clock speed, instead of forking out the cash and resources to get it done, they diverted funds away from the Amiga division and started an MSDOS PC compatible division.
- They also delayed the introduction of the A1200 and A4000 until the A600 was ready and discovered that the A600 was a total flop anyway and should never have been introduced into the AGA lineup.
There's probably more reasons but those are the main ones.
I'm not entirely sure all the delay of the 1200 and 4000 was the 600. larger delay was the shelving of the 3000+ project which was the first machine being made with AGA chips and the DSP chip that ended up being taken out in the final products and used by Apple in their AV line of Macs.
Also not sure the MS-DOS machines diverted any funds. Those to my recolection were a project by Commodore Europe and once released sold very well over there due to the good name Commodore had. Since most develpment of Amiga was in the US I doubt this had much effect on anything other than more income into Commodore as a whole.
That Commodore sat on the same basic custom chips capabilities with just small tweaks from the release of the A1000 in '85 through the 600 in '92 is probably the biggest influence in the decline.
It could be said they lost a lot of direction and made a lot of bad decisions along the way that all served in the small parts to slow down the company to a crawl.
Abandoning the Pet line for the home business when the Vic-20 came out, which disconnected Commodore from Business computing for so long that even when Amiga came out no one considered Commodore seriously as a business machine.
Instead of keeping to the plan of a low cost Sinclair competition, the expanding of the C116 into the Plus/4 was a huge bungle and went from a machine that would sell ($50-75 for a color computer with chicklet keys compares favorably with a black and white computer with membrane keyboard that shuts off the display when software loads) to a machine priced the same as the C64 that no one wanted.
Never releasing the Commodore LCD whcih at the time would have easily been the best portable computer around (and vaulted Commodore back into businesses) and instead leaving that market to Tandy with the Model 100.
Not having a plan to position CDTV. It compared favorably with the CDi and already had a library of software available which should have been a clear advantage over Phillips.
Cost reducing the 500 into the more expensive to make 600. Commodore cost reduced the C64 and 128 so they should have known how to cost reduce something by then.
Bungling numerous attempts at deals with the Japanese.
Not accepting a deal with Sun to have them sell Amiga 300UX machines as low end Sun workstations only to later enter into a deal with Nettek to let them rebadge Amigas as Video Toasters.
Not releasing the CD32 sooner so its sales might have had an impact and the money might have helped keep Commodore solvent.
Also have to remember things are a lot different now. After sale of a computer the company didnt make much money at all on it besides some software they sold under their brand and the hardware enhancements they sold until a new model came out. There was no app store to make Commodore able to keep making a continual income once the computer was in the customers hands.