There is a very good reason for having a dangling object hanging from the mirror being an MOT failure !
It is not really a failure because it obstructs your view, it is a failure because it is
dangerous, especially for cyclists and pedestrians.
Why it is dangerous, ( as 99% of drivers do not know) is, if there is something constantly moving within your field of view and after some time your brain gets used to it and the space the stupid dangly thing moves in and it starts to get ignored by your brain it also ignores everything that happens in that space which includes pedestrians, cyclists or an obstruction.
Think about it.
There should be a ban on them and points plus a fine for all the idiots that have them!