It turns out that Samsung is pre-loading the Galaxy S21 with Google Messages in most markets, as Engadget learned from an interview with Google's Hiroshi Lockheimer yesterday. By doing so, Samsung joins other OEMs like OnePlus, LG, and Motorola in using Google's messaging app by default. Unfortunately, we in the US are stuck with the regular Samsung Messages.
From the timestamped interview link above, you can hear Lockheimer discuss the reasoning behind using Google Messages on a Samsung phone. He says that it feels "native" because it's slightly modified to pair better with One UI, both aesthetically and functionally. Lockheimer goes on to mention that the United States, along with "some exceptions," will instead get the regular Samsung Messages app. It's worth noting that Samsung added support for Google's RCS to Samsung Messages last month, so there's no real difference in functionality.
It's unclear why Samsung decided to stick with Samsung Messages stateside, but perhaps US S21 owners will be able to sideload the Samsung-customized Google Messages APK.
- Engadget (YouTube)