In my opinion, this update is beneficial for the following reasons:
-It's now more organized
-Players are able to view previous reports and see which ones have gone unnoticed, thus allowing them to bump the threads and notify staff that said threads have gone unnoticed
-Completely removes the problem of comments on reports, which was problematic since reports were often breeding grounds for drama
The argument regarding being unable to add additional evidence is one that was used a while back when this change was also considered, and to be honest it's so situational that I've never seen it occur. Most players tend to add all the evidence that they have, and if another player has additional evidence then because of how many reports there are, they usually just create another thread, completely unaware that the same player had already been reported.
If offenders wish to view evidence that was used against them, then staff can still provide it. The threads are not deleted; they're inaccessible to uninvolved users.
Overall, I reckon this is a positive change :)