Welcome to the community!
My opinion is that the Librem 5’s design philosophy is based around the existing Linux ecosystem. Purism is too small a company to develop and support every app in a core set of phone apps (where ‘core’ is in the eye of the beholder). So what happens may depend more on what is happening in the wider Linux ecosystem.
Looking at the phone right now, that is absolutely not what is happening. Phone calls are in one app. Text messages are in a different app. Emails are in a third app. (On the other hand, you may be right that they all share Contacts, that are maintained by a fourth app.)
On the plus side, it’s all open source, so if someone wants to hack that altogether into one monolithic app then lack of source is not stopping them.
I can see advantages of what you are suggesting e.g. I remember that a person sent me something but I can’t remember whether it was via text or via email, and if I just want to check the content of that communication, I don’t really care how it arrived.
My take is that at this stage, it is more appropriate to be filling in the gaps in the individual apps e.g. getting MMS support in the messaging app (which will also need to support an extensible set of internet-based messaging services) and e.g. my pet peeve, lack of POP support in the email app and e.g. more work could be done in the phone calls app.