The end of a text is already final enough, so a period doesn’t add any functionality that way unless you somehow believe someone got raptured mid sentence and still hit send.

Sure it does. It disambiguates sentences from questions.

If you received a text that read "Buy bread", there are multiple ways to interpret that. If I assume the lack of a period is a statement, then I would assume the person is instructing me to buy some bread at the store. But if there were a question mark at the end, the meaning changes - they are asking if they need to buy bread, or perhaps asking if I have already bought bread or am planning to buy bread.

Now, if you are familiar with the person on the other end of the text, you might already have the necessary context to parse it out without the need for formal punctuation, but if it's someone you maybe don't text very often or a complete stranger, wouldn't it be nice to know for sure if they were making a statement or asking a question, definitively and clearly?

Adding an intentional period removes any doubt that it might have been a question. It eliminates the possibility that the person on the other end simply forgot to add a question mark while typing out a hasty text at the grocery store.

You might argue that period or no, the message got across just fine, and that assuming no punctuation is the same as having a period, but that's not the point. The point is that the period does add functionality, it just isn't functionality that you deem useful.

I certainly wouldn't want to replace the effectiveness of proper punctuation in favor of having the period be reduced to a means of signing off my messages in a passive aggressive manner. I can do that all by myself, thank you very much.