Assumption 3: Our storage needs will continue to increase over time.
The thought of ever filling 30TB of storage space today may seem like a fantasy, but that may not be the case in the future. Basically, this comes down to the computing corollaries of Parkinson's law: "Data expands to fill the space available for storage," and "Storage requirements will increase to meet storage capacity." In other words, the more space we're given, the more space we'll fill up.
Until the year 2010, I was still (somehow) using a 60GB hard drive that I had for ages. The only reason I ended up swapping to a new drive was because the disk crashed. I can't imagine how I managed on a 60GB drive, but I did. I swapped to a 1.5TB drive thereafter, and within a year, I had just about maxed out my space on it. My storage capacity increased by a factor of 25, and yet within one year I had adjusted my habits to accommodate the new amount of free space.
This effect isn't just observed on the consumer's side of the equation. Game developers adopt similar practices - as we moved from floppy to CD to DVD to digital download, games have grown larger and larger. With the advent of 4K resolution, we can expect this trend to continue unabated. The same can be said for movies and TV shows - the jump in resolution translates into tremendous file size increases. Compression technology just isn't advancing as fast as graphics, because it simply doesn't have to. Necessity is the mother of invention, and we don't need better compression when we can easily get more storage space.
So where does that leave us?
If we continue our doubling trend, by 2016, we'll have 16TB drives, 32TB drives by 2018, and by 2020, 64TB behemoths. By then, 30TB will still be respectable, but it certainly will no longer be impressive.
Of course, we're talking about the largest drives commercially available to consumers. How would 30TB compare to the norm? A quick look at Amazon's best-selling hard drives tells us that the average consumer currently has a 1TB disk. That's one-eighth the size of the current largest commercially available drive. If we assume the same scaling, then 8TB drives will be the norm by 2020.
But when would 30TB become the norm? In 2022, the largest drives will be 128TB, and in 2024, 256TB - at which point 32TB drives will be average (one-eighth of 256).
As for whether or not my friend will need more storage space by 2020... Well, by Parkinson's Law, he probably will have filled out most of it by then. Five years worth of games, movies, TV shows... it'll add up, especially if he has no reason to delete anything. But just like I made due with a 60GB drive, that doesn't necessarily mean he'll need more space - he can simply adapt to what he has.
Now, how accurate are our answers? I suppose time will tell. What's important is that we didn't just hazard a random guess; we thought critically, based ourselves on data and facts, and put our precious little skull-dwelling jelly-monster to work. Perhaps you tackled this problem from a different angle and arrived at a different answer - and if you did, please do share your methods and results. As long as we're thinking, we're living.