I read what he wrote differently

>> If only there had been a popular 6809-based computer with better hardware than the CoCo.

> That wasn't really the issue, the C64 wasn't much better

the 6809 was a really nice "ultimate" evolution of the 8-bit CPU. The C-64 was just another primitive 6502. So I think GP meant "I wish there was a 6809 I could get that wasn't the CoCo, because I want a 6809!" rather than meaning that the hardware doomed the CoCo or something.