Umm, by statute good tradelines aren't required to fall off after some period of time. In theory, they remain forever, but each bureau has their own rules they follow.
Anytime baddies go away, that's good for you. Losing a good TL might hurt a little, but I suspect the baddies going away will do more good than any loss you'll see from losing the good TL.
Do you have a current auto loan in good standing? Mortgage? Credit card or two? Having these, keeping credit card utilization low, and keeping payments current will keep everything in a good state. If you only have one CC with a low limit, you might think about applying for a second card. Having an AMEX or Discover card, anecdotally speaking, seems to have a more powerful positive affect on your credit than a VISA or MC. Adding one can help a lot.