We were very happy when we heard Samsung had finally relented and allowed users to remap the Bixby button to another app.
Unfortunately, it seems Samsung still wanted to restrict competition to its Bixby voice assistant, and would not allow one to set the button to Cortana or Google Assistant for example.
It did not take long to work around this issue, however, with XDA-Developers releasing a small stub app called Bixby Button Assistant Remapper which can be selected as a target application, and which will then automatically run your chosen voice assistant (after selecting the default once).
The app works with Cortana and Google Assistant, but unfortunately not Alexa.
Read the full instructions and download the APK from XDA-Developers here.