Welcome to Data Archiving 2.0
So when the first vendors started their products due to regulatory controls/pressure/mandates their major appliances that data was moved onto was WORM or WORM-like. The customers have continued over the past 7-10 years to just add more, of the same vendor's kit, and now have PB's of data/email/IM's & more than likely files archived.
That's great if you're the archiving SW vendor or the HW disk storage vendor but what happens when your CIO falls out of love with the HW vendor? Yup that's right, it's Nightmare on Migration Street. The saving grace is that storage area networks have incredibly high bandwidth.
Pssst here's a little secret - the Cluds ain't got it, they're limited to POTS/Fibre/Common Carriers. So where you have pretty damn decent in-house bandwidth to migrate, the Clud doesn't have it, so you're as sticky as a customer as those on WORM or WORM-like HW.
Migration is more painful that a Root Canal done rectally, and is more of an inexact science to boot. Welcome to Computings New Nightmare on Storage Street!