You're right. But JW wants to be here, his market post-injury isn't likely to be what it would've been, and the Saints want him back. I strongly suspect that the Saints know what it will take to get him back under contract.
And there's nothing saying the Saints couldn't have both if they wanted. I don't think Teddy is guaranteed a starting job next year. I don't think Jameis is guaranteed to be ready by week 1 next year either. This may be a situation where we need two QBs that can start.