Using wireless to control processes or devices is still being evaluated. Just as with earlier technologies (PLC, for instance), the trust factor simply isn't there - this is human nature when dealing with new technologies. We have done several small installations, and I recently completed a POC at a WWTP that exceeded our expectations.
To the interference issue: most wireless technologies use spread spectrum or OFDM modulation techniques, which are highly resistant to RFI. We were also concerned about this and our concerns were unfounded, particularly in the 5GHz bands. Locally, our sensor network (802.15 based) is in an electrically noisy space, but this has had no discernible effect data transmission or quality. While there are many very solid 802.15 based sensors (Bluetooth, WiHART) available, these systems require protocol translation and integration into a plant-wide 802.3 or 802.11 network, requiring interposing equipment and added cost. The tipping point will be when instrument vendors adopt the 802.11 standard as a native communication method such as several have done using 802.3 Ethernet.
To the maintenance issue: A wired system goes down and it takes days, possibly weeks to troubleshoot, repair and test; a wireless system can be back up in hours. There are no wires or conduit to maintain or protect (or deteriorate) and while wired systems connect point-to-point, wireless is "unbounded" and can connect to multiple points - at a fraction of the cost! The new generation of operators is very comfortable with wireless, in fact, they are requesting it.
In my POC I used Emerson wireless equipment "in place of wiring" and saved the authority $25,000 over a wired system. We now have a working (Emerson) sensor network at a remote site and Wifi coverage within the footprint between the process and control building - the operators love it. In Monroe County, NY, the authority is using Emerson wireless equipment "in place of wiring" and they have saved "hundreds of thousands of dollars". Speaking with their I&C guru, he related to me that the system worked perfectly even though it was covered by two feet of snow. He also related that he is using wireless as the primary means of communication with his remote emergency showers - as a backup to his wiring, which he does not trust. He plans on continuing to use wireless primary sensors to transmit 4-20ma signals to the gateway and then to the plant network and SCADA - "in place of wiring". Other installations I've done were specifically designed to replace wiring, and they are solid - saved a ton of money, too. This technology exists at the enterprise and instrument levels and is collapsing the traditional communication model, eliminating much of the costly wiring and specialized labor required to install and maintain it. Our new SCADA upgrade will integrate wireless technology as means for bringing new wireless sensors (4-20ma data) into the network. I would posit that FO is required as a backbone, at a minimum, but copper wiring is slowly becoming obsolete.
The same arguments were made against PLCs, against wired Ethernet, against SCADA, even against HART, which Emerson has reconfigured as WiHART. The cost of wireless averages 75% less than a similar wired system; it is more flexible and reliable, and has considerably less down time than a damaged wired system While I agree that existing systems will stay wired (until they fail), the future is a wireless world.