Were is the evidence that the aircraft moving to TT are '15 years old'? One would suspect that the transfer plan would leverage the youngest out of the mix, with the oldest to be returned to lessors or sold as the MAX's arrive?
There is obviously economic benefits by leveraging a single type for the group that supports not just engineering, but training and other support provisions. Tiger is small, and will not become the behemoth like Jetstar - so it makes sense for the small operator to take advantage of group scale and economies to optimise profitability.