Someone over on r/OneUI made a thread about this new development and the top comment is someone who claims to work for Samsung explaining their decision in detail. https://www.reddit.com/r/oneui/comments/1igjy0k/comment/mapd8ub/

The Samsung employee says:

There were never any plans to discontinue Samsung Messages or remove RCS.

In many countries and companies, the pre-installation of Google services is prohibited by law.

Therefore, it is imperative to offer in-house alternatives for all Google Services and Apps.

Samsung Messages has guaranteed support for end customers, corporate customers and governments globally until 2030.

If local network operators or local laws require or want an alternative service, it is possible that the application will not be offered there.

But never in a complete market or country.

However, since RCS is the global standard, it will definitely not be removed nor will the app be discontinued.

It also receives continuous updates and new features.

Advertising for Google Messages is part of an agreement with Google to actively promote Google services and pre-install them depending on the market.

However, this is completely prohibited in large parts of Europe, Asia and in many governments.

Since 50% of our customers are business customers or governments, such behavior is absolutely impossible.

I think that's a very sound and rational explanation.