They are a destination for many travelers. They will obviously take some revenue from existing businesses in Lafayette but it will be minimal compared to the amount of new revenue they will bring to the parish.
For example. No one in my family heads west to Houston or beyond without planning a stop at Buc-ee's in Baytown. If not for that Baytown would just be another blip on the map with no particular reason to stop.
And the #1 reason travelers like Buc-ee's..... the bathrooms. Just ask your wife or kids.