If you use a system after a while, it's expected for the HDD to be filled with lots of data from installed applications, system files and even your personal data such as photos/movies etc.
This is standard across any computer I believe, a simple clean installation should bring your system back to new and fast again.
Probably when that time happens, you are already considering getting yourself a new computer with better specs.
I'm using 15" Macbook Pro Early 2011 and quite happy with it other than the known graphic card issue.
Hardware parts are easily replaceable(DIY), self replaced my superdrive with the built-in HDD and installed an SSD at the main HDD slot, fusioned the 2 drives and got myself a DIY fusion drive. Works much faster since then compared to before(5400rpm HDD)
Apple is still selling these 2012 MBP(only 13" for now) -
Configure - Apple Store (Singapore)