If someone discovers a free, unlimited energy source, launching a 51% attack would be just a matter of seconds. And that is exactly what has happened.
Free energy source alone can't help! It will only be possible if some billionaire can gather enough ASICs to generate 51% of total mining power! That's an investment of few billion dollars. So good luck with that!
POS is more user friendly I would say because you wouldn't need to go through all those mining rig setups. You just need to buy some pre specified amount of coins and stake it through a pool or an eligible wallet. So it makes the process easy for all! Also it doesn't need to use electricity so it is more environmental friendly as well.
But your concept of 51% attack is flawed!