As plants develop and grow over time, they must continuously deal with the need to maintain and update their assets.  Process control systems are one of those assets that must be evaluated regularly for any potential risk to operations as it approaches or passes obsolesce.  While control systems can, and often do last well beyond their expected lifecycle, the question becomes what is the risk when an HMI fails, but there is no replacement available for it?  Or if there is a replacement available, how long will it take to receive, configure, and install the new HMI?  You can keep replacements in stock, but if there are several platforms involved, the costs to stock spares can start to add up, and this may not be the best use of your dollars.  Keeping up with current HMI platforms ensures not only that replacement parts are readily available, but in the case of thin client technology makes replacing an HMI on the floor almost trivial.