However, the limit on how good a bike is are defined by materials and geometry together. With strong, tight and shock-absorbing enough materials, you could theoretically create a race-quality bike in a form-factor like this.
When or if such things will appear is hard to say. The Bike Friday seems to be at the quality of a decent road bike but foldable into a suitcase (they've had the same basic design for twenty years so twenty years of materials improvement might create a pretty extraordinary thing).
See: https://bikefriday.com/ (relative to other comments, slightly larger than Brompton but I think more of a bike that really can be your only bike).
I have a Bike Friday (and I even visited the factory in Eugene OR to pick it up, which was fun). There's a clear difference between it and a Brompton, slow-fold vs. quick, essentially. I have gone on great plane trips with my Friday, but it wouldn't be very practical for taking on public transit, and it doesn't fold up much without coming apart, and that takes tools and time.
Bike Friday are pretty popular in the Willamette Valley, I guess some local pride. My neighbors have a couple and taken them all over the world, I think they put something like 60K miles on them by now. They look pretty goofy but seem to do the job.
I can't see myself on one of these things but I never had a commute where it would be necessary, in a pinch maybe it's the lesser evil.
However, the limit on how good a bike is are defined by materials and geometry together. With strong, tight and shock-absorbing enough materials, you could theoretically create a race-quality bike in a form-factor like this.
When or if such things will appear is hard to say. The Bike Friday seems to be at the quality of a decent road bike but foldable into a suitcase (they've had the same basic design for twenty years so twenty years of materials improvement might create a pretty extraordinary thing).
See: https://bikefriday.com/ (relative to other comments, slightly larger than Brompton but I think more of a bike that really can be your only bike).