At the time, Apple was still quite open, especially with the II series but much less so with the Mac. The book for my Apple IIe actually has full schematics of the system along with the theory of operation and everything you need to design your own peripherals and program it. Steve Jobs' vision of the Mac was a computer that was an appliance, one you could simply take out of the box, plug it in and use it. Not my cup of tea, but there is certainly a market for that. People like my mother for example are not computer savvy at all but need to use a computer for work. She doesn't care how it works internally as long as it does what she needs. I was never a fan of Macs back then but as I play with them now it's undeniable that like the Amiga, the 68k Macs were far ahead of the PC clones of the era, though correspondingly more expensive.
I suspect that had development of the Amiga continued, the hardware would have made the same inevitable shift toward commodity PC components as Apple. Processors became more and more powerful and GPUs and audio boards became commodity items eliminating the need for special custom chips. Development would have focused on the operating system as the hardware became more generic. The availability of software for the platform is the make or break aspect as it is for any platform and there is no telling whether Commodore could have maintained a niche in audio/video production and games as Apple largely has in the graphic arts and desktop publishing industries.
Computers just aren't as exciting anymore as they used to be. The tech has matured and everything has largely plateaued. Even low end PCs are now more powerful than needed by the vast majority of users need. Rather than bringing new features and capabilities, new versions of software generally just move everything around and make the UI uglier, conforming to the latest fads. I don't even upgrade anymore in most cases because rather than being better, I've too frequently found that new versions take away features I used and hid other things.
