My niece was given the little red bicycle my mom learned to ride a bike on after my dad refinished it, the frame was great, just the aesthetics needed touching up. This bike is in the running for 60 or 70+ years, which is probably not an uncommon occurrence..

Over the summer my brother lent me his bike to use for a month or two. It is a 1974 Gitane racing/road bike, so nearly 40 years old, and it was still in fantastic shape. Practically zero visible rust, frame was still decently straight, etc. (I did have to replace a plastic gear in the derailleur)

No kidding. I've worked with a few people who have done bicycle advocacy in West Africa (through the awesome https://bikesnotbombs.org/). They learned that West Africa is awash in cheap used bike frames - it's about the only part of the bike that is eternally repairable. Instead, the finer parts eventually get chewed up and need to be replaced. Things like ball bearings, derailleurs, chains, and tires eventually die. Wheels last a long time, but also have a limited life.

This project (and other ones I've seen, like bamboo bikes) generally only create bike frames, the one part that won't really help the developing world.

To the cardboard bike's credit, the wheels are pretty cool. Find a way to make brake pads, chains, tubes, and tires from locally sourced or recycled materials, and you'll really be on to something!