In short, I don’t think it’s realistic even if technically possible.
One might argue that a person (or small conspiracy of people) might try to hoard/delete bitcoins in order to destroy the currency. Given that the quantity supplied of bitcoins is capped (by mathematical definition), one might imagine an attempt to buy it up and then delete it.
Let’s say a future multi-billionaire goes insane and tries to hoard all 21,000,000 bitcoins using his life savings. It’s not so easy. First, he would have to start bidding up the price. He must enter the market like anyone else but must offer the highest price – by far. Of course, he might succeed in buying a large percentage of bitcoins using the most attractive price. However, some sellers still won’t sell. Additionally, the currency can be broken down to the hundred millionths (i.e. make a transaction worth 0.00000001 BTC), so buying “enough” isn’t realistic either. Since the market won’t run out of money, an attempt to buy it all up will simply lead to massive losses on behalf of the buyer. A disincentive to even try is built in. This is why the rich man would have to be insane to make a go at it. It’s financial suicide.
A key factor that will determine the long-term viability of bitcoin will be supplier acceptance. If suppliers desire (and are permitted by governments) to accept bitcoin in exchange for goods and services, the currency has a future. It’s off to a good start in that regard, but broader acceptance needs to continue. The ability to pay one’s utilities for example, would be noteworthy progress.