For many Android users, texting is still a very commonly used practice of communication even with the number of instant messaging apps out there. Hangouts being one of the most popular eventually received the capability to merge the Hangouts messages with the SMS and MMS ability so both of those functions were available in one app. As it stands, any Hangouts user can opt to enable Hangouts as their default SMS and MMS app, which not only consolidates things down to one app, meaning less space used on your phone, but it also makes for quite the convenient setup as you only have to manage one app for all of your messaging needs whether it's text based or not.
A new rumor suggests that Hangouts may end up having the capability to send and receive text messages and act as your default text app removed, although whether or not this is certain is still unclear at this time. So far there has been no confirmation from Google, but they do have their own standalone texting app with Messenger, so it's not entirely outlandish to think they would want a more focused experience for the user in both situations. Should this happen and Google actually does remove the text capability from Hangouts, those who were previously using Hangouts as the default app for text messages will end up having to nail down another app to take over the SMS/MMS functions.
This could be Google's Messenger or the stock texting app, but in either case, Hangouts would simply be just for Hangouts messages and voice/video calls. Right now there is no telling when Google would make these changes, if at all, but in the event that this feature change is in the works it would certainly warrant an announcement from Google on the matter, possibly through a popup message inside of Hangouts as that would be the easiest way to alert people to a major feature removal and minimize the amount of users who may end up missing it. According to the details, a new version of Hangouts is said to be going through internal testing at Google which makes the suggestion to users to give Messenger a try, so the changes could already be far along and on their way to the public soon, and this would likely serve as the notification popup to alert users to the changes.