Agreed. Custom design is still prohibitively expensive, and most often not needed. That being said, older process nodes are getting cheaper to the point where the manufacturing cost is feasible. The problem remains EDA tooling, which may very well be the majority of the cost. Without more competition in that space, I don't see Cadence/Synopsys/Siemens bringing their prices down any time soon.
The older nodes have been cheap enough for quite a while now. Folks like Xfab have nodes where you can generate chips for about the same price as you can an injection mold.
Lots of guitar pedals would be interesting and they can use old school 1um+ technologies that tolerate +-9V. Low power stuff actually likes larger transistors as they have much smaller leakage.
There's lots of interesting stuff to do if you can sink the NRE. The problem is that the super expensive CAD tools don't let you amortize something that isn't a million+ units.
Besides 130 nm is plenty good for a lot of industrial applications.
That said, despite what some ASIC houses will tell you, I haven't seen the costs narrowing on commercial ASIC design to the point that realizing an SoC would be a substantial cost savings on an aggressively cost-optimized microcontroller-based design or IoT design. Being able to do a commercially viable small-to-medium-sized ASIC for a design that ships, say, less than 2M/year, would change a lot of things. It would be very interesting if that were the case in 5-10 years - what do you see from the IC design perspective?