I think the fundamental problem of Symbolics was their strategy as a systems company. They were a half-assed hardware company that could not keep up in price or performance and didn't even bother to let others develop system software for it, and they were a half-assed software company that didn't bother to port their software to other computers (a $10,000 plug-in board for a Macintosh does not count as a port).
They were also a pretty good computer graphics and computer algebra software company, and both those divisions got sold off and continued their work for a number of years after Symbolics itself went out of business.
It's interesting to note that by the time of Symbolics' demise (I'm placing it at 1993ish, although that's probably not accurate), even IBM realized that developing systems as integrated software/hardware architectures was not feasible.
You must be using a very narrow definition of "hardware architecture". In the 68K era Apple used to custom-design pretty much everything interesting in the box except the processor and memory. The IIfx was perhaps the most extreme example (several custom ASICs and two 6502-based I/O coprocessors, IIRC). For the Newton we even designed parts of the processor (e.g., the MMU).
Let's take your example of the IIfx: off the shelf 68k processor on NuBus. ASICs and I/O coprocessors are just devices. Granted I didn't know about the custom processor work on the Newton, but if IIfx counts as a new hardware architecture, then so does the x86 move from ISA and 8259A to PCI-E and APIC. And I don't think it makes sense to argue that today's Macs have anything resembling a custom architecture.
Symbolics' systems and the first AS/400 models (the last new hardware/software architecture developed by IBM) are in a totally different class.
They were also a pretty good computer graphics and computer algebra software company, and both those divisions got sold off and continued their work for a number of years after Symbolics itself went out of business.
It's interesting to note that by the time of Symbolics' demise (I'm placing it at 1993ish, although that's probably not accurate), even IBM realized that developing systems as integrated software/hardware architectures was not feasible.