There is a strength ceiling that your body can achieve, but it's hard to get there, I think.
Your advancement won't stay linear either. At some point it'll take longer and longer to advance, and your advancements will be smaller and smaller. When you just start, you can make encredible gains, but when you get better, you'll have to work harder and harder for those.
If you're done with the pulldown machine, you can move on to chinups or pullups, and then to weighted versions of those
