Merlin still claim to be "second only to Disney" remember? With that in mind surely they should be creating attractions better than Universal, regardless of attendance? Hell, even on par with them would be good.
And Transformers cost under $100m, with a much bigger building, with the ride on two floors. The scale of it is huge, where as this is quite small. If they made Transformers the size of DBGT, the cost probably wouldn't have been too much different.
But ultimately, Terror Castle at Brean is a lot scarier than DBGT, so that's £30m well spent Merlin!
Correct me if i'm wrong but Merlins claims to be second to Disney are more aimed internally / to shareholders regarding visitor numbers rather to the every day guest visiting for a day out at one of their theme parks..
Guest feedback for DBGT seems to be very good (saw most people giving it 8/9's out of 10 during the tech rehearsal surveys) and that was over me 4/5 rides on it during that period - so if the GP love it then it is money well spent.