This isnt a PS+ issue its a platform issue in general. The PS3's value will diminish over time just like any other platform that is being phased out. Software support will slow down as well. Theres nothing Sony can do about it.
If the consumer is not willing to move on to PS4, PS5 or VITA etc. Then they can unsubscribe from PS+. Thats the trade off with PS+ free games...thats also what makes it possible that people can get all those free games for the price of one over a year period.
At the end of the day Sony cant just hand out free games...i dont think anyone expects them too really.
Edit: Not too mention how unfair it would be for those of us who are paying full price for the games. You need to look at what the actual goal of the PS+ service is.