In my humble opinion, the reluctance to update from Windows XP to another version falls within four categories:
1) People Don't Like Change;
2) People can't afford to upgrade the OS and hardware at the same time (or period);
3) Availability of updated and/or Compatibility and Stability with XP apps; and
4) Businesses are the majority of users.
People are extremely adverse to change, especially when they have no control over it. It's the fear of the unknown and how it will negatively affect their job as opposed to the positive the change might bring. To alleviate this fear, you must convince them why the change is necessary, how it will benefit them, and make them feel comfortable through it.
I've found users are apprehensive about the stability of the new software and whether it will be compatible with the current programs (Apps) and hardware they have. They are concerned they will have to relearn programs they have become familiar with using (not including updates to the MS programs) and the cost associated with having to update to newer versions of those programs. In a lot of consumer, and probably some business cases, the user needs to purchase a newer PC (I know people that still use PCs with 512 MB RAM and 40 GB hard drives). The money and training time required to upgrade can become expensive for a business or consumer, especially since a number of us have not recovered from the economic crash of 2008!
To give you some perspectives to this poll, check out this CNET article: Windows XP won't die without a fight
Why do you think XP is so resilient, even as support is coming to an end?
-- Businesses are the majority of users.
-- People in general just love XP.
-- Windows 7 and 8 are not worthy of upgrading to.
-- People don't like change.
-- Pirated XP versions make up most of those users.
-- If it isn't broken, why fix it?
-- People can't afford to upgrade the OS and hardware at the same time.
-- People have apps that run only on XP.
-- Other reasons. (What do you think?)
Place your votes here

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