I personally agree that wireless is not so much as to take the place of wires as it is to measure all the stuff that was not measure before: missing measurements that were not included when the plant was orignally built. I see wires (hardwired or digital bus networking) used for the stuff "on the P&ID" complemented by wireless sensor networking for everything "beyond the P&ID". That is, the primary layer of automation is the wired ICSS (DCS+SIS) while wireless for pervasive sensing is a second layer of automation. See further explanation in these articles:
NEW: Beyond the Control Room
www.ceasiamag.com/.../10658
Second Layer of Automation
www.ceasiamag.com/.../10354
Maintenance with a Hart
www.ceasiamag.com/.../9894
Smart Sensing: Situational Awareness
www.iaasiaonline.com/more.php;cat=im
Are you sending people in harm's way?
www.iaasiaonline.com/more.php;cat=ii
Wireless for Asset Uptime
www.ceasiamag.com/.../8689
For a new plant, this has to be planned from the very beginning of a project