One of the abandoned apps in the Nextcloud store, spotted that one earlier and wondered what happened. Would be interesting to see that replacing all the work done on the native functionality.
I was wondering about this too, and there are three possibilities I can think of:
The existing Nextcloud Mail app will be rebuilt on top of Roundcube.
A relatively “simple” official app is released that integrates Roundcube more or less as is to Nextcloud, and Nextcloud Mail continues to be developed independently.
Nextcloud Mail will be replaced by this “simple” official Roundcube app. Very unlikely as the UI is completely different from Nextcloud’s and Nextcloud-specific integrations are missing.
I personally hope for option 1, however, I think it is more likely to be option 2.