I agree that properly designed thumb safeties do not slow the draw or in any way hurt.Trying to keep this as short as possible-
The only thing that keeps a pistol wthout a thumb safety from firing when you reholster is that nothing pulls the trigger. It is a single point of failure. A properly designed thumb safety (ref: 1911 style) in no way hurts- it’s not slower, or hard to learn to use subconsciously; but it does add a huge margin of safety.
I also agree that the only thing keeping a striker fired pistol from firing when being re-holstered if debris enters the trigger guard is a manual safety.
However, re-holstering a hammer fired pistol with the hammer down and the thumb covering the hammer will provide substantial notice of (and impediment to) trigger movement.
-J