While I highly doubt the 2019/2020/2021 Rams 12" screens will get the UC5 update, I bet they will have a 12" (or rough equivalent) UC5 in 2022 or so. I cannot see them dropping the Ram down to 10", leaving the earlier 2019-2021s with a larger screen (even if the software is better). Also, Ford and GM now have similar sized screens (or are coming out with them shortly). To go smaller, from a marketing standpoint, doesn't make much sense.
The 10" does make sense in the Jeep, Pacifica, etc., because they never had the 12" screen. They had the 8.4" prior. FCA may have made a bit of a design/engineering boo-boo, putting a "unicorn" screen into the Ram that none of their other cars will get. Also makes sense as to why it may be the last to be updated.
Regardless, despite its SiriusXM shortcomings compared to the 8.4 radio, I do really like my 12" screen.