Wasn't it because Jack Tramiel was too cheap and just wanted to make use of his existing MS BASIC licence?
A more interesting possibility is the post-Tramiel Commodore including a ROM version of GEOS from 1986 onward, and selling it on cartridge form to existing customers.
Other possibilities:
* Launch Amiga 2000 and 500 in 1985 instead of 1000.
* Eschew Amiga completely, in favor of the Commodore 900 with Coherent. Instead of Amiga silicon, ship with a "VIC-III" for graphics and two SIDs for stereo 6-channel sound.