If Bitcoin were to go the Proof of Stake option (using sidechains perhaps), one of the problem people have with the idea is that it will become much less secure because it would be very cheap to perform a 51% attack.
Some PoS coins boast that you need more than 51% of the coin to control/destroy the normal functioning of the network, but we'll just focus on 51% now.
Your question is based upon a false assumption that a 51% attack on a PoS coin requires one to purchase 51% of coins.
Certain variations of PoS have already been successfully attacked with only 5% stake -
https://bitcointalk.org/index.php?topic=897493.0, and more sophisticated variants of PoS likely need around 25% stake. This stake doesn't need to be purchased either but could be temporarily leased, held by an early stakeholder, or come from a compromised large stakeholder. With PoW there are physical considerations and expenses that often need to be met before performing a 51% attack which raises the security of bitcoin.
This being said, I like TaPoS and would like it added as a PoW/TapoS sidechain or as a security layer within a wallet ontop of PoW for bitcoin. We should be considering ideas to make bitcoin more secure and not less secure just in the name of cutting down on an electric bill.