The monitor would last forever--unlike cathode-ray tube (CRT) monitors of old, the TFT panels these PCs use won't degrade over time. They won't go blurry, for example. Their brightness might dim over the space of a few years if they use cold-cathode backlighting, but the trend nowadays is for LED backlighting, which should retain its luminosity for the life of the machine.
As for upgrade possibilities, the reality is that it's rare for businesses to change hardware within computers. It's just not an efficient use of staff time and PCs are treated as commodity items, to be used until they die and then disposed of. Any upgrades are limited to RAM or, even less frequently, hard disks. In many cases all-in-one PCs manufacturers make the RAM easy to upgrade, and if the computer is used as a cloud terminal then running out of storage will not be an issue. Nothing other than the OS is stored locally.
Cloud terminals really would be commodity units that could be swapped in and out as needed, just like mainframe terminals of old. If one died, the tech support operative could get another out of storage and just plug in the same keyboard and mouse. There would be no need to take data off the machines when they reach the end of their lives because the data would be stored in the cloud. Users wouldn't even be aware they were using a new computer because their desktop environment would be stored in the cloud.
Of course, all of this is pure speculation--fantasy, even. Businesses are still wary about working within the cloud, and there's no sign of an end of the security deadlock that's stopping many businesses moving over to the cloud.
But most of all, the history of corporate IT is one of buying PC boxes, and that's not going to change overnight. The IBM PC set a pattern that we've yet to break, despite 30 years having passed.
It's fun to see how lots of different computing technologies are converging at the moment to made cloud computing a possibility, if not a reality. As they said in "The Six Million Dollar Man," we have the technology. However, how we use it is down to how quickly we can abandon our prejudices.
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