I think the only reason to do a custom IC, for now, is if you can't achieve your performance, power, area or price targets with FPGAs, microcontrollers, or other discrete components. It's still impractically expensive unless your volumes are enormous or your margins are very high. Even with a reasonable complex feature set, I think you can do better with individual components. You can get relatively cheap ARM cores, FPGAs, DSPs, microcontrollers, etc. I don't often see companies doing custom ICs for internal use.

That being said, I think the economies are changing. If you don't need a cutting edge process, the tapeout costs continue to come down. The problem that remains, though, is licensing cost of EDA tooling. There is a little bit of competition in that space, but not much. It is growing though, so hopefully that brings prices down.