I was seeing possibilities dating all the way back to late 2015 that Samsung and providers might make the Marshmallow update available for Galaxy Grand Primes. Is this still a possibility or would I have to pay off this phone and get a different one?
Contrary to popular belief, firmware updates are developed by the manufacturer, not the carrier. Manufacturers and carriers do not provide estimated release dates. The only available information is contained in the Software updates page.
Currently, the Grand Prime is not listed on the Software updates page, so no update is currently being developed. There is no way of knowing whether there will be an update in the future, other than continuing to check the Software updates page.