Re: Surely this is a Customer side problem?
No, it is a condition fabricated by M$ for obvious financial reasons.
Leaving inactive accounts in place is a perfectly good data archiving solution.
The data is NOT "dumped on someone's PC" it is on Onedrive, meaning that it is stored in the cloud.
It may even be on your on-prem, privately managed cloud, but because it does not have a liscence from microsoft for the Onedrive user account, Onedrive will delete it.
There is little justification or benefit from doing anything more than archiving the data, without a clear requirement or need.
In the future, if someone comes along and needs to answer specific questions, say they want audit proof for a SOX compliance filing, then that person can apply the knowledge they have to go through the data.
The data archiving guy has no hope of being able to categorize that data properly.