That's a tough question. It sounds like VW will be replacing the Passat in the US with a bigger sedan that costs a bit less than the current Passat.
Cheaper is good, unless you like the smaller size, and unless you are concerned that the interior materials will be downgraded in the new sedan. VW is known for their interiors though so I doubt that the new car's interior will be lacking.
The other concern is that the first cars from the new factory may be problematic. Nissan had lots of issues with the first year or two batch of cars from their Canton Mississippi factory when it opened a few years back.
And it's not clear to me exactly when the Chattanooga plant will go into production. They are still building it.
Dealers want to sell what they have in inventory, so even if they knew that the Passat was going away next month, they may not tell you.
A lease would let you walk away in a few years if you wanted to get into the new sedan. And if you didn't like the new sedan, you could buy the leased Passat.
But leasing is like renting and you're stuck with mileage limits and it's hard to get rid of the car early. And you'll have no equity in a leased car, so you have to be careful not to pay too much for the lease.