I think I’m right in saying GT Sport was spun off of GT7 development because they realised how long it was going to take. GT proper has thousands of car models and many hadn’t been updated in years. I remember people back in GT6 complaining that some car models were looking pretty dated. That obviously won’t cut it in the 4K era.
They branched GT Sport off of the main GT7 codebase to make a stop gap. That suggests they didn’t stop working on GT7. Obviously GT Sport will have taken some of that time, but surely all the cars and track they’ve made can be incorporated somehow.
I will be very sad if they don’t keep the improvements from GT Sport.