The Falcon 9 is a heavily-tested rocket that is very different to the Starship prototypes. Different engines, different structural design, different type of fuel, ... The least-tested component is the Crew Dragon module, but one of those has already been to the ISS, and has been through both on-ground and in-flight abort testing. There have been zero Falcon 9 Block-5 launch failures (there was one Block-4 RUD). There was one engine shutdown due to some cleaning fluid igniting (it was left after refurbishment, processes changed to prevent this). The other engines took up the slack and the mission was a success (they failed to retrieve the booster, but that's not part of the mission objectives).
There is always a risk when putting people on top of explosives and sending them up to orbit. I'm confident that those risks have been extensively mitigated for the Demo-2 mission.
I imagine the SpaceX engineers are hoping they do learn things from the Demo-2 mission. It's just very unlikely to be directly related to Starship. One of the things they're explicitly hoping to learn is how much degradation the solar panels suffer while attached to the ISS.