Online mailboxes in Office 365 offer essentially limitless “free” storage. This sounds like a great deal, and maybe it is. Still, stuffing that online mailbox with the digital landfill of a decade-plus of Exchange and PST data is not a great idea.
The reason why you’re saving this data is the concern that someday, somewhere, sometime, somebody’s going to need to retrieve it. That will play-out very badly if you’re trying to search a multi-terabyte online repository through a web browser – you’d actually be better off if you didn’t keep it in the first place, not to mention you might be unable to find it anyway. A much better strategy is an offline storage strategy that is easily managed and won’t clog-up your shiny new Office 365 environment.