Apple deliberately slowing down phones after a couple months of use?

Updating a phone to the latest OS is one thing.....that phone running the OS stable is another thing.

I had a 3GS and i updated it to iOS made me want to rip my hair out. I did a hard reset, but that only helped for a couple hours.

My mom had a 3GS, she never updated the OS and only use FB, text messaging, Pandora and made calls on it. That phone started running like molasses about 4 months ago (close to 2 years of use). I did a hard reset for her and a week later, the phone started running slow again. I strongly believe these phone are made to start slowing down after a couple months of use. so that you'll upgrade. She finally upgraded to the 4s, because she hated how slow the phone got.

Am I losing my mind or is this a thing?