I remember as a child being stuck in the back seat of the car for over three hours in 35°C heat just to get through Millau.
The town is at the bottom of a very steep valley and it is very difficult to avoid (this involves extremely steep and narrow farm roads that are difficult to navigate without a small 4x4).
How has the bypass caused Millau to change?
Has it prospered or faded now that there is no through-traffic?
Given its proximity to the Parc Natonal des Grandes Causses and Gorges du Tarn it really didn't have to worry about that. It is a very touristic area.
Tourism is good, and the area is renowned for trail running, gravel biking (UCI World Series), mountain biking and paragliding.
The viaduct has made some villages on the plateaus much more accessible. Small industrial businesses have set up shop.
The only thing that sucks is that the little railway line will probably never reopen.
It has lost through traffic but gained quite some tourism to see the bridge, it's a win situation