> Most computers in the world don't need the ability to have their operating system changed or run custom user built software.

That's not true? Nearly every laptop, desktop, and server can trivially run whatever OS+software, of course, but also currently at least virtually all Android devices can run arbitrary user built/supplied software (I grant that, unfortunately, replacing the OS is often harder). Embedded stuff varies, but frequently anything capable of running a general purpose OS (which IMHO is generally the line to call it a "computer") can have it replaced.