Back-up tools for Office 365

There is not a good story around this at present.  Most tools are extensions from the on-prem counterpart.  So you can find good SharePoint backups, but mail is usually non-existent, or not that flexible, and vice versa.  Further with the integration, you have complications of some data, in regards to how to restore it (o365 Groups, Planner, etc).  Beyond SharePoint and Exchange, you have stuff like data in Azure AD, Yammer, O365 Video, and PowerBI which as far as I know are not addressed by any product at present.  Most if not all of them fall short in terms of eDiscovery as well.  You also have to consider how the backup products keep up with changes to the service as Microsoft updates API's or adds services that complicate the backup and restore processes. 

Based on the size of your customers, I find it hard that you are able to sell an additional service at all.  The value prop is almost non-existant.   The native tools you have can protect against accidental deletion in most cases.  If they are having issues with people manually cleaning up recoverable items folders, or SharePoint recycle bins, there is a larger issue at play as that is willfully circumventing the tools meant to protect from these scenarios.  You can leverage the legal hold features if their tenant supports them to provide an additional layer or protection as well.  I would not do this on every SharePoint site, but with high value content, you could do that and documents are copied into a hidden library.

You are not without "Backups" entirely when relying on Microsoft.  SharePoint is backed up every 12 hours and data kept for 14 days.  You don't get item level restores, but you can request a site collection be restored.  For Exchange, it can be scary to think Microsoft does not use backups, but they do keep lagged copies, so that in the event of an issue, they can swap your databases to a time previous when an issue was detected.  You can't request this change directly, but it's there. 

The real benefit of a 3rd party,  is in the peace of mind it gives some people.  But in almost all cases just a little education up front can avoid most of this complication.   Let's face it whether it's Microsoft, Google, Amazon or any other cloud service, you have to trust them with your data and that their processes and systems will protect you from data loss.  I find Microsoft services to be very reliable, and I generally feel more confident about the data there than I do in a lot of customers local SharePoint deployments.