It is really moot and dumb to speculate about HSR fares so many years before they become available. Maybe there will be attractive family fares… who knows.
If you decide to drive, fine; that is your decision, and nobody should object to it.
FWIW, through-checking of baggage from a train station to a plane and inverse has been around for more than two decades in Switzerland (called
Fly-Baggage). It is now 24 hour before the flight leaves that you have to check in; a few years ago, when the SBB still had decent baggage service, you could check in, and your baggage was on the same train(s) as you were travelling to the airport.
In fact, United Express and other airlines woould be happy to hand over that not so profitable short distance business to the HSR operator. (another FWIW: since the high speed line between Frankfurt and Köln opened, there are no more (Lufthansa) flights between Stuttgart and Frankfurt and Stuttgart and Köln; you still can book, and you will have LH flight numbers, but you will sit in a reserved space of the high-speed trains, and your checked baggage will be on the same train as well.