Think of it this way: Anyone can attempt to execute a 51% attack. However, if you own less than half the hashrate, then it becomes exponentially less probable the more the blocks. However, you might stand lucky with 30% of the hashrate and reorg, say, the past 2 blocks. That is quite probable to happen in fact, you have 44.6% chance. What is exponentially improbable, as time goes by, is to maintain a chain with more work than the rest of the network, therefore to continue launching the attack forever.

By the time you will have started reorging the 5th to last block, the rest of the network will be way ahead of you, so you will have to work even more to keep up with the new blocks, and it goes on and on. The honest miners will build faster than the attacker, and outpace him.

That was the first quiz, and you all did pretty well. Class passed! More Bitcoin questions will come in the next days. Whoever has thought of a good question can drop me a PM, and I'll take it into consideration. Have a good week.