I think you are missing the point of the strategy of playstation plus. Like some said the games that appear are often at least a year old or sometimes less than a year old but still the reason why it works is because games have their selling time and then take a serious dive. Games like Little Big Planet Karting sold poorly so then it is put free because the months pass and it still sells crap. Do you understand? So Sony does not loose money by putting it for free as its selling meagerly or nothing at all. Instead people that get it for free will try it and then two things can happen:
1. They will buy dlc for the game gladly, especially if they enjoyed them
2. They now could be willing to buy day 1 a potential Little Big Planet Karting 2. This might not have ever happened if the game was never given for free as the game was poorly advertised and or the reviews might have scared customers away.
You have to take those two given points into consideration. It is a way to try to bring greater awareness for a game that failed comercialy but that could now spike up interest for a potential sequel and profit through dlc.
There are also games like Bioshock Infinite that sold well you could say but that its current sales have taken a huge nose dive. It is a scenario where it sales are now probably meager and its fanbase unlikely to expand. The only way for this to happen is to give it to people who are not willing to pay for it for free and as they play they might learn that they like the game and then might go on to buy not only dlc for it but maybe even Bioshock 1 and 2 if they never played them. For 2K, they probably figure that they can only benefit from putting on playstation plus.
Now let me tell you. Grand Theft Auto V will not be appearing until its sales decrease significantly. And think about it, call of duty is never offered. Why? Because these are games that have a different sales trayectory and continue to sell in a way and for a long period of time that most other game do not. Sony and developers are happy that you try their games as you said that you would only buy Dark Souls 2 day one. Maybe you will buy dlc for these free games and or fall in love with one of the free games you get.
To end, I would agree with you if Sony put for free a game on its launch day or withing 1, 2, or 3 months of its release. That would negatively impact sales. Half a year later is when everyone that really wanted the game already bought it. That game will then sell very little.
