I think there are many factors that cause companies to either change or stay with their equipment types.
1. Changing platforms requires more spares to be held
2. Re-training of engineers
3. cost is one obvious factor in changing platforms, however, this has to be factored in the decision due to the above 2.
4. Many companies especially in the US or US owned in other countries like to be patriotic, with large corporations this seems to be more important than cost.
5. Upgrading even if using the latest equipment from the sme manufacturer is not always tht cost effective, many upgrades will still require a lot of engineering input & RW's approach to a prolific ever increasing versions & their seeminly reluctance to allow their IDE's to be used on legacy equipment makesit a little painful, I say this with a little reservation as I have not been in the loop for a couple of years, just what I seem to notice from other posts.
Many HMI's can be replaced with other types without a lot of effort. I replaced 6 off PV550's with Beijers E series, the mounting hole was different but we had a number of adapter plates made, in actual fact the Beijers had an adapter plate to convert from the older series to the E series that worked, however, these werenot stainless so our local metal basher made some in stainless & the cost was only slightly higher than the adapter plates you could buy.
Although not overly complicated (but probably considered middle complexity) It only took me a day by having two laptops & checking the configuration between the two, as the other 5 were almost identical with minor variations I could do perhaps 3 a day, in all, the cost saving compared to either keep maintaining the old panels & upgrading to the Beijers, the apparently more reliable platform, the additional benefit of colour & better resolution graphics extra features, lower cost of the units & so on it was a no brainer. It was even better for our maintenance staff, should a unit fail, we had a set of USB sticks with the program & recipes on, just a matter of getting a new unit, plug in the stick, copy it across & fit it.