Will i5s be good in 5 + years?

Gaming wise, doubtful. Games are the few pieces of software that will push hardware. Even the Core2Quads are almost to the point of being outdated.

I wouldn't recommend an i7, not when you can get a Xeon E3 1230/1231 v3 or better for less.

Five years is a LONG time in top end PCs. Will software need more than 4 threads execution units? Not required, but will probably benefit from them. Just as you can run top end games on an G3258, you will still be able to run with 4 threads in the future. Will you get beaten by people with newer hardware -- yes.

Before that your display will probably have to be replaced, your RAM will probably want to be upgraded, you will have gone through 3 graphics cards. You can keep a 57 chevy running well forever, but it won't have all the airbags of a new Lexus ...

It all depends on which way the development of software will be going. In some cases threading and multicore will help to make use of more advanced physics, NPC (AI) and other complex action that could elevate a game, but not everything will be able to make use of more threads/core.

The trend you can see for lately are that game will be designed for portability across different plattform like PS4, Xbox One and mobile system like Android/iPhone. It will mean that gaming system on PC probably will stay on the same level as other hardware feature on Console, with some minor adjustment for more power on visual content. Steam machines could change this as those devices are more similar to regular computer systems in their build, but will still be limited to the more compact form factor (=less heat).

A "locked" processor will of cause be limited to what kind of multi it has to change as opposed to an unlocked "K" processor from Intel with an accessible multiplier which can be set to a higher clock. There is also a limit on a system level for how one could push this to compensate for faster processing in the future.

As an example you could take a look back in time for 5 and 10 year ago and try to make an estimate for yourself from that time forward to what you will find on the market today on both the development of application, type of games and their use of hardware and how the general development so far have been in between those points for 5 to 10 year. Could you still use a CPU from that time today or a video card and be happy with it? How much would you need to "upgrade" to get to that point and is it even possible (DDR>DDR2>DDR3 and soon DDR4)?

I think you now get my point that it is A) not realistic to think that you will have a processor that would still be midrange for games, if the development of processor would change. B) It is more then just a processor that matters for gaming system or even in a general computer as SSD have changed how one could design a computer vs HD and we are using less and less Optical Disk Drive (DVD/BlueRay) and more and more Internet for storage solution (like Steam/Origin etc).

What everyone is trying to say is we really don't have a clue, but overall, any modern CPU will most likely be outdated in five years for gaming. So, save the money, get the i5, and save up for a new build in a few years.

No, an i7 isn't worth buying, unless you are an overclocker and want a k series chip.