Cold Storage Add-on for Pro accounts.

Why would you want them to spend a boatload of monet and manpower to put together a low-demand product that would be next to impossible to make a profit on when stacked up against Amazon Glacier? Even then, you would be looking at $200-$250 per month if memory serves me well. To get near that pricing, Dropbox would be spinning their wheels for a handful of customers that wouldn't be making them much money, if any. Meanwhile that same amount of storage arranged, packaged, and sold as dropbox would produce their normal profits without having to bring on new skillsets. It would be foolish of them to try to jump into that game. If they're gonna spend money I'd rather have them bring on some people that could really improve the GUI and user experience of their current products. 

I ran into a similar situation 10-12 years ago with my Plex server because I had 20TB for that while also having another 10TB of photography work to protect. The solution now is the same as then. Download microsoft sync toy, buy a second set of drives and a NAS unit, and just copy them over. Then take that thing to a safe place. Safe deposit box, would be best. But just having them at a friend's house in their safe that lives a couple hours away is better and gets them geographically separated. If at a friend's you can even connect it to their network and map a drive to them over VPN. For 50TB you can do this for about $1200 one time if you buy the cheapest you can get. As a service though, they have to consider the value of their space that can't be used for anything else, the fact that they have to maintain it. If a drive fails, they have to repair it. They have to secure it and provide bandwidth for it. And for all of that, even with say, a rock solid 100M upload speed, it's gonna take 51 days to upload it.

At least you'll think so at first, but you'll find that on a sustained upload like that, the bandwidth will fluctuate on all but commercial dedicated connections, you'll need upload during that time for yourself to use your connection those two months, and when carriers see long sustained utilization you start to get throttled...and the longer you're sustained, the slower it gets. Because their networks aren't designed for that. When a connection isn't a dedicated connection it's what's called "shared" and the capacity offered to you and everyone else in the neighborhood combined is about 10-15 times  more than what's actually on the line. Having worked in telecom for 21 years I've seen enough to give an educated guess that your initial upload would take roughly 5 months to complete if left non-stop on a 1 gigx 100m resi fiber connection. You could copy these drives in less than a day depending on your interfaces, drive them elsewhere, set it up, come home and stream a movie. 
You would be saving $1500 the first year and $3000 per year after AND have read-write access when you feel like it since anytime you read from glacier you'll be charged even more.