Old is relative, and is heavily related to what you would have to change.

For example, our old 1970’s tandem has French thread BBs, a method of securing the eccentric that will never be good, and a seat tube diameter such that I had to custom modify an off the shelf seat tube. I got it when I couldn’t afford more and spread the upgrade money over several years. While it got us into tandeming, and while parts are technically available, the cost to update or even maintain (as replacements are needed) all those things very quickly ends up at the purchase price of a newer nicer bike with a better frame.

Our second bike (early 2000’s) is great for me because it fits my spare parts and lets me have linear pull brakes plus an Arai drum. A rear disk would not allow that. On the other hand, if someone really wanted say disk brakes front and rear with a composite fork it would mean new hubs, fork, and brakes (I do have a rear disk mount). That quickly becomes a >$1000 project. If that was my goal, I should have gone just a few years newer and gotten that up front.