If they've shot themselves in the foot here, it wasn't by charging for preferred seating, it was by charging more for an upgradeable fare. If they weren't charging for preferred seating, you still would've booked in T+, but they wouldn't have gotten that extra ~$25 from you.

I still think the preferred seating idea may hurt them, but for other reasons. I buy T+ partly to be able to choose a good seat. If the best seats are always blocked off, then once I pass the status qualification threshold, I might as well just buy T tickets and take my chances selecting seats at check-in time.