Exchange database dismounted due to NTFS file extent limit reached – unexpected outage

What version of Windows Server and what version of Windows Server was the NTFS filesystem created with if it has been migrated from a previous machine?
What NTFS cluster size are you using? It's recommended to use a cluster size of 64K for Exchange databases and t-logs. Is this direct attached storage or does the datastore reside on a SAN/NAS?

There has been very little technology changes in the way of managing Exchange databases all the way back to the Exchange 5.5 days. The method back then, and the method to this very day is still to scale mailbox servers horizontally and balance the mailboxes among them in order to decrease individual database file sizes (and therefore decreasing the number of extents which the file is comprised of).

From what I understand, you can either continue using NTFS and fight the extent limit battle with enabling large file record segments and ensuring correct cluster size, or use ReFS like you mentioned.

So, in conclusion if your goal is to utilize existing storage hardware without Microsoft causing you to invest more capital than you already have, go with ReFS. It has all-around better large file storing capability and offers better data integrity compared to NTFS.

If other file systems were supported such as ZFS, BTRFS or XFS I would be recommending one of those instead.