Alton towers cannot cut down on themeing without a serious amount of their fan base getting all mad. They did that with Air in 2002 after they went over budget and it is still mentioned by many today, how Air could be so much better with the themeing that was planned for it.
Plus with a ride that uses a story like this, and is based on the actual towers themselves, they really cannot afford to cut corners on the themeing, when that is a massive part of the ride. Themeing can make a bad ride look good. Im not saying Thirteen is going to be bad, but it sure as hell isnt going to be on par with a couple of the other coasters, so it needs all the help it can get themeing wise. Themeing can work magic, look at Indiana Jones and the temple of Peril at Disneyland Paris, the basic ride is just an Intamin remodel of a Pinfari looper, but the themeing makes it look near unrecognisable.