LTO8 is about 20TB per tape so not that many tapes, even if they are a hundred bucks a pop.
The real cost, I think, would be in paying people on site to physically secure those tapes off-site. I suspect that's why VFEmail didn't have off-site physical backups; it was a relatively small operation, with servers in datacenters on multiple continents, and probably didn't have the budget to pay people to physically load blank tapes and put filled ones in storage.
It's feasible for a handful of administrators to run lots of virtual servers in datacenters around the world. It's considerably more expensive as soon as on-site human labor gets involved.
And running those sorts of backups remotely probably wouldn't have been feasible either, due to latency and bandwidth constraints.
That doesn't mean data like this shouldn't have off-site physical backups, of course. I just think the economics are difficult. How much more can you charge your customers to cover those backups without having an unsustainable fraction of them switch to competing services? Users historically have not shown much willingness to pay extra for security.