The vast majority of people have been crying out for allocated seating. A lot of people choose not to use easyJet because of the current boarding process.
The trial is showing that the passengers like it and they are able to board quicker since nobody is faffing about with what seat to use. You also do not have to pay for an allocated seat as one will be given to you regardless but no guarantee that it would be one that you want unless you pay a small fee. I understand plus holders will not be charged to select any seat and will still be boarded first.
easyJet is not making a mistake, it's just for once listening to its customers and giving them the option if they wish to choose it.