I know it's impractical and can't be done within probably even a day nor a week, but what would happen if someone very rich would like to spend ~1 billion on bitcoins and buy it all?
At first glance looks like it would be the end. Thinking a bit more it suggests that the later bitcoins would be so expensive that even he couldn't afford it, but he would have so much that could make a huge profit out of it.
If there's someone more literate than me in economics, I'd love a detailed and argumented response.
It is simply not possible to buy it all even with an infinite amount of money. The simple reason is that not all bitcoins are for sale. Currently, the only ones that can be purchased are on the exchanges. If someone spent a large amount of money on the exchange, the exchange rate would rise and this would probably lead to other coins being available on the exchange, but it would never lead to all the coins being available on the exchange.