It seems, like there’s a need of extra abstraction layer over smarty in order to be less dependent on php/smarty versions change. (this is a joke )
Yet another moment is that there’s a php7.3 beta available already, so it is possible such sutuation, when TB still won’t be compatible with the deprecated php 7.2
Both things are important equaly: if you break compatibility then you risk to loose a lot of clients, from another hand, keeping situation frozen again brings risk to loose prospective clients and those ones, who have decided to upgrade/move to more modern solutions.
So, TB devteam, being limited in resources, has faced difficult challenge - how to keep balance between moving forward and holding backward compatibility for the sake of conservative lients.
As it seems to me, i would choose the way of moving toward php 7.2/7.3 and newer smarty as a priority, keeping support for the key customers, fixing open issues and holding backward compatibility. Unfortunately, it is much easier to give such advices rather than predict which direction will be right.