We can see how this is unlikely to work when the dog is in a highly aroused state of fear/anxiety because the learning functions simply do not operate in these situations. We (humans or dogs) find it very difficult to learn new things when we are worried/anxious or stressed. The fear quite literally overules any other information. The technique of exposing the person or dog to lots of the fearful stimuli is called 'flooding' and is considered out dated and potentially harmful in the treatment of phobias.
In theory though, when we are dealing with a low-level fear, you would expect that gradually over time as nothing terrible happens, the dog might start to relax and learn that *tennis balls/children/footballs/car travel/raindrops/plastic bags etc* are not actually that bad. I can only really speak from personal experience here - but I have only seen this happen in young puppies under about 4 months. After that, I've found that it takes actual desensitisation and counter conditioning to really impact on the dog's perceptions and fears.
For example: Farah came to me quite scared of feet moving near her and especially over her. Lifting your foot to put on a sock results in a cowering little dog. After a couple of months it became clear that nothing was really shifting - so I used a combination of limited exposure to feet with clicker training to help her interact with feet. Farah learned that instead of feet being a potential signal for pain, they were just another object to interact with and earn 'clicks' from.
So, if your dog is nervous around toys/plastic bags/the hoover/anything at all - do think about finding a way to help them relax and reframe the objects, rather than just expecting them to get used to it!
