Do not take it personally but this question gets asked a lot. Not necessarily here but I've seen or heard it a lot. The appropriate answer is similar to what Brain responds to Pinky - the same thing that happens when every new system comes out. More games are exclusive to the new system, while the old system with the bigger install base still gets a number of good games. After a couple of years, when the install base is big enough, those big name games are only produced on the new gen systems while some games are still produced on both and even less are only produced on the previous gen system.
If you really want an idea, see what happened to the PS1 when the PS2 came out and the PS2 when the PS3 came out. The PS2 lasted longer (I believe) than the PS1. The PS3 will be around for at least 3-5 more years. Now, how long games you want will still be coming out for it are another question. It is still one of the best all-purpose systems around. It blows away the Xbone as an all-in-one multimedia center IMO.