If this is a true statement then as an IT technician myself, I can confirm that this is a smart decision.
I know that some may create accounts, be active for a while, then stop logging in. Others may have created an account, but were too nervous about joining such a community, so they stopped using it very shortly after they join.
Assuming your observations are correct and there have been ten thousand or so accounts removed, from a security standpoint, that’s ten thousand site vulnerabilities removed. Regardless of what this website is all about, there are bad guys out there (from every country on this planet) that are trying to cyber attack anything and everything they possibly can. In this case, there are now ten thousand less ways for the bad guys to attack this site.
Also, data storage. Profile information have got to be stored on a hard drive somewhere…and those hard drives need to be backed up. Even if those ten thousand accounts did not have much information about themselves on their profiles, there is still information that needs to be stored and backed up. Removing that many inactive accounts frees up so much hard drive space which also means that there is less data that needs to be backed up.