Ok, but you can also use "thanks" as a goodbye. Consider ending a phone call, maybe "thanks" is for whatever they called about, but it's effectively "goodbye" before hanging up.

you can use it in place of goodbye, but you would be using it with different meaning. this is illustrated by the fact that there are certain situations in which you would never end an interaction with "thanks", but you might "goodbye". for example ending a call with a lover