[1] They decided the OS wasn't that important... sure, let Microsoft own that. The CPU architecture wasn't that important... sure, let Intel own that. Just glue a bunch of off-the-shelf logic together for the rest. After all, the real value is in the IBM logo on the box. What could possibly go wrong?
[2] Sure, they were calling the shots for the 8086 and (mostly) 80286 generations. They tried to rectify their mistake over half a decade later with the PS/2 (i.e. proprietary everything) but by then it was too late. Compaq and the clone makers owned the market from the 80386 on.
[3] Out of the frying pan and into the fire?