It seems to be some pro choice v no choice.
With the option to do it, the choice is up to the individual, with the no option then all that GT Sport achievements, be that cars accrued, and/or liveries/decals whatever, is lost. I would think twice about putting too much time into designing liveries if PD don't value the work and time that has gone into them.
For all we know, the idea of transferable cars accrued, and/or liveries/decals may have always been the plan, and both games have been designed with that in mind. If that were not the case, then it could be a feature that could be easily added, or indeed, not possible at all, only PD know. And I would only want the feature if it were easily added btw. PD don't seem to need too many reasons to take their time making their games, and I would like GT7 as soon as possible. But again, nobody knows.
PD/Sony know how much data is being stored on their servers. Any new servers would be designed with that in mind, and taking into account how much data new features may take up. And they would do it with the knowledge that they would plan for 100% migration, but not everyone would take advantage of the feature. The sooner they get people mainly using GT7, the quicker they could scale back and close down the GT Sport servers.
Are PD looking for a restricted number!
PD kept saying that GT Sport cars and tracks were designed with the option to easily use in a future game. Again, only they know how easy it is, or isn't, or if it is even possible, to transfer the assets from GT Sport to GT7.
While it may sound nice to limit what could be transferred if it were possible, firstly, why put restrictions on other than a car is in GT Sport and not in GT7 other than the car not exist