Q: My MacBook Pro was built in Mid 2009, I understand that i will not be able to get the next OS Sierra.  I just put a new 700GB hard drive, plus i added a new battery to include a new CD/DVD player.  I spent over $400 bucks. Do I have to by a new Mac?

We never know for sure what Apple plans on doing until they do it, we can only guess based on past behavior (which can always change without warning).  To differing degrees Apple and other software developers will support old OSX versions for a while.  For example, the current iTunes supports OSX back to Mountain Lion 10.8.5 (earlier this year they cut off older systems).  On that basis I would guess as long as you are willing to upgrade to El Capitan you can probably do iTunes upgrades for the next 4 years.  However, that isn't to say that if Apple introduces some new feature that it may require Sierra for that feature to work, particularly if Apple deems some hardware feature necessary to use it.  Again it is a matter of what you need and when.  People who always want the newest software and don't want to have to take care of a computer or think about it too much get the newest things (Apple's target market).  People who aren't even aware of the world changing around them can use older equipment as longs as they stay in a time bubble (like somebody with a 2006 Mac and OSX 10.6 and an iPhone 4 with iOS7 and iTunes 11 and a printer they bought 5 years ago).  People who want to keep old equipment going but also try to use it in a modern context need to learn how to do so.

For as long as they are available, apps and operating systems are not the same.

that rep has provided you with an irresponsible disservice if he said the computer would be "useless after a year"

You will still receive them for as long as Apple supply them.