What 'both sides'?
One side is the powerful serial rapist like Cosby that took 60 victims before anybody came forward on the one side, where a system like this would have gotten him caught considerably faster, and lacking it nearly left him getting away with it. The other side is the one off rapist where if a witness decides to seal the report but may have not done so if it wasn't an option, then the rapist gets away. I see two different situations and this system being a huge help in one and a huge problem in the other.
You and I agree that the identity of reported victims can be kept confidential until trial, so the issue of privacy and protection has been addressed.
In theory sure, but I don't think that is possible if the accused is questioned. If he's questioned about a rape he did, he'll know exactly who the victim is. If he is questioned on a false allegation, there is still a good chance he'll know who is accusing him (people only have so many enemies who would be likely to do so).
All that's left is the matter of keeping the actual reports confidential, which IMO just makes future rapes more likely.
The benefit is that the option to do so increase the number of such reports and the likelihood of them existing. I totally agree that it would be nice to have them out in the open and widely known by all police everywhere, but that can't happen if the victims never make the reports in the first place.
Marie was the guy's first victim. Had her report been taken seriously the cops might have gotten lucky and caught the guy before he attacked the second or third victims.
Her complaint should have been taken seriously, if it wasn't. All complaints should be taken seriously, investigated, properly scrutinized and determined if they have merit through the collection of evidence for and against the allegation being true.
Even if she wasn't the first, a situation where there "may have been reports from earlier victims that were unsealed because there were enough of them" means there would have been multiple victims before a proper investigation was carried out. That's unacceptable.
Only if those victims opted to seal their reports. Those who seal their reports are those who are unlikely to come forward otherwise.