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).