I think some folks are missing the point here.
A DCS encompasses not just the 'controller' part of the system but the entire architecture all the way from enterprise-level down to remote IO and serial interface options. HMI connectivity is built in, comms are redundant and secure. Scan time is guranteed and there are a ton of diagnostics.
I have yet to find an HMI/Controller combination offered by a PLC vendor that even come close to the functionality and stability of a DCS.
I work on DCS systems from the 1970s (TDC2000) to right now (CS3000 R3.08) and they have come a long, long way, as have PLCs.
A DCS is aimed at a market where 10,000 IO and 40+ operator stations is normal. A PLC and its associated comm structure would struggle to accommodate such an undertaking, coupled with its limited redundancy options I doubt it would ever seriously take the place of dedicated DCS systems from Honeywell, Emerson or Yokogawa.
I'm not trying to flame anyone in the slightest.
I have come across some installations where PLCs have been used and a DCS should have been.
Every one was a train wreck.
Now PLCs are wayyy more common because they are perfectly suitable in 90% of industrial applications and generally very cost-effective.
Its the other 10% thats the territory of a DCS and a DCS alone.
Of course a DCS costs about 10 times as much too