Hi,
first of all, this feature is here cca over 8 years, it was created before new Joomla! ACL system was created. It was added because of need of users - users wanted to have such type of rights (for users only). So implementing is not the right word.
Sorry, but I cannot apologize for creating component for my self, releasing it for free for public, adding new features asked by users and maintaining it for public.
If you look at Phoca Download implementation on Phoca Download site:
https://www.phoca.cz/phocadownload - you will see that I (for myself) don't use any ACL - all files are public - I use cca 15% of features implemented in Phoca Download - all other features were added because of users need and this same is the users rights rule.
I fully understand, that listing all users can be a problem on sites where there is a lot of users, but the project is open and this feature is used by a lot of users, I cannot just remove it because this then will break features on many thousands of sites.
So in fact there are two problems to solve:
a) there is a group of users who uses and needs this feature and there are some users who get problems with this feature
b) you cannot implement "popup method" as the select box is multiple select box
So changing this feature is limited at least of two factors - users who use them, technical problem to select more users
The advantage of the open software is, everybody can modify it for own needs so If you modify it the way that:
a) the feature of selecting users will be still available (because it is used for many thousands of users)
b) there will be possible to select the users multiple way
c) and it will not limit your site (no problems with loading large list of users)
and let me know the solution, I will be first who will take it seriously and will try to implement into component as default feature.
As I wrote, I fully understand the problems on your site but we are placed between two factors and we need to find some compromise between them and of course we cannot break backward compatibility
And of course, I am open to all possible solutions of this problem and I there will be some, then it is really no problem to implement it.
Thank you for understanding.
Jan