Hi,
interesting topic. If I can add to it, I would say we see much more dramatic changes in the field within last 5 to 6 years than in last 20. The biggest change is the standardisation of technology, something what happened in internet many many years ago. You can see following criteria becomes more and more adopted and what is more important required by the users
common communication protocol with open specification (OPC UA in Europe and MQTT in USA)
operating system independence (windows, Linux, Mac OS)
mobile devices support
web based (HTML5, SVG, JS)
data-mining
etc.
Great example of this achievement and what it can bring to end users is mySCADA, and you can see even big brands like Siemens do follow. For example Siemens adopted the OPC UA protocol for its PLCs this year.
Link is here: mySCADA