When a positive TL drops off your CR, then yes your score will drop. This is one of the unfortunate realities of credit reporting.
By law, the CRAs and furnishers are limited to how long they can report adverse TLs. Once it's too old, it has to drop.
However, there is no legal requirement on how long positive TLs have to remain on your report. Once the OCs stop reporting, like when you pay off a car loan or PIF and close a CC, there's no telling how long it will remain. Might stay until you die, or it might disappear in an "oops". More than a few people have reported CSC apparently arbitrarily deleting positive TLs whenever a consumer disputed something.