that game is over for me once I've launched the rocket

Ahh, well that definitely isn't the case for me! I usually keep playing long after I've launched the first rocket. For me, launching the rocket is a somewhat arbitrary "ending"; it's a good objective for people to focus on - especially new players - but I don't think anything really changes before or after the rocket launch in terms of gameplay loop (and there's no narrative to change). Just like before the rocket launch, there are still things to optimise, new ways to build, etc, (some of which are supported by the science you get from launching rockets, in fact).

I suppose it partially comes down to whether you're an objective-driven player or someone who enjoys the process. For me, it's all about the process/journey, and the objectives are more of a guide than anything. If the objectives are complete and I'm still enjoying the process, and there's still room for me to enjoy the process, then I'll keep playing.