15 years ago it was likely a 7 speed. In that same space todays bikes have 11-12 gears. Additionally chains and gear teeth can wear together way past the desi

15 years ago it was likely a 7 speed. In that same space todays bikes have 11-12 gears. Additionally chains and gear teeth can wear together way past the designed for tolerances. So over 15 years things might work, but then you have to replace the chain, cassette, and chainrings. Replacing any piece alone would likely make things MUCH worse. Generally if you ride daily it's worth replacing a chain annually, or at least checking for "stretch", which is actually wear. Chains are MUCH cheaper to replace than chainrings and cassettes.