Here's the thing.
Is Bixby even that good to warrant its own dedicated button (that's now unmappable)?
Google Assistant is nice but I only use it to check the weather and create reminders. I haven't really fluffed much with AI-based assistants because they all have a ways to go before I find them truly useful without feeling awkward while using them. From what I've seen with Bixby, it doesn't seem to have that one killer feature that would make me go "Yes, I want this!". Maybe that might change when I give it a go, but as it stands right now, I'm not entirely convinced that Bixby is worth having its own dedicated button.
That's just my own 2 cents, though.