Like the title states, there's a thread at XDA claiming the Bixby button can be remapped to launch Google Assistant. I can't confirm until I have my GS8+. The thread can be found at the link below.
https://www.xda-developers.com/how-...-the-galaxy-s8-s8-to-launch-google-assistant/
https://www.xda-developers.com/how-...-the-galaxy-s8-s8-to-launch-google-assistant/