The Amiga’s heavy focus on TV-friendly timings went deep into the specialised chips to a point it was difficult to upgrade without losing compatibility. Because of that lack of modularity Commodore had to spend more resources to develop improved machines than its competitors. It was not an obvious mistake then, and it’s not clear now what they could have done to better compete with PCs and Macs.
They could have made much simpler “productivity Amigas” with plain VGA-like graphics to leverage its non gaming software market, at the expense of only having minimal graphics and sound support. There was, IIRC, one, made by a third party that lacked the Amiga chipset, but running the Amiga OS. If they could push something as cheap to make as a Mac LC, they’d have a much more attractive offering for businesses.
True, but "Amiga made lots of assumptions about the use that ended up..." seemed an odd way to phrase it, given that it was originally designed as a games machine, although the plans were expanded later in development.
As I understand it, its custom chips were a brilliantly clever solution to a problem that existed at the time. It couldn't be called a mistake, because they couldn't see into the future. As a games machine, the Amiga ended up hamstrung by those same custom chips because they weren't the right architecture for Doom and all the 3D games that followed it. That made no difference to its productivity software though, did it?