The last system I had where I didn't complete virtually every game I bought was probably SNES. Back in those days, the idea of 'completing' a game was somewhat vague. There were no trophies or percentages. Many games had an ending but if you took every warp in SMB, did it count as completing it? Several others, like Tetris, don't ever end. I also just bought what ever cheap stuff I could find, used at a game store or garage sale or something.

As my free time dwindled as I got older and had a full time job and kids, I consciously only got a few games at a time and finished those before I moved on. There are rare exceptions where I just couldn't get into a massive game, like Ni No Kuni or Xenoblade Chronicles X, but I cultivated my games strictly enough that I enjoyed most of what I got.