Of course I'll cast my -1 vote as well.
I'll add to point 1:If the data contained in the model is all of basic types or implicitly shared types then it's possible to already use the current framework without ever triggering any expensive* copy
, to point 2:There's no clarity on how to support different types in different roles would we need an additional template argument for each used role beyond Qt::UserRole? is so the problems above grow exponentially with the roles
And to point 3:No possible implementation for a reliable dataChanged emission if model internals are exposed directly
*expensive = a copy that implies more that a:memcpy of less than 65bits and an increase/decrease of a numeric reference counter