I still remember when you didn't even have to open up the "MBP" and could replace the battery directly from the bottom. Even had green led indicator to show how much charge it had directly on the battery case.
Old batteries wouldn't be useable at half the age most modern batteries to start swelling
Then again, I was probably harder on the battery in those days than I am now, with my laptop often plugged in. I supposed my 2011 Air lated until at least 2018 on the original battery though.
Remember the glowing light near the laptop latch that would slowly swell to tell you that your laptop was asleep?
The Unibody had this lovely latch mechanism that also allowed you to easily replace your hard drive and/or RAM.
God, I miss those days. :(