[Psychlist]

A missing top tube is much easier to deal with than a missing down tube. Lots of bikes have pushed that a very long way. But the main forces there are twisting the pedals against the handlebars and using the front brake, which the downtube is very involved with.

You could build that bike out of steel tubing and it would be rideable, as I said, but insofar as it's broken it's broken at the missing down tube.

Slingshot, for example, had no down tube just a wire and that was a bit notorious for being squirrely in the steering.

You could definitely build a bike like that that was quite rigid, it would just be heavy or expensive or both. A decent carbon layup, for example, might bring it back to the chain forces going through the seatstays being the main issue. But that's something you'd want to analyse a lot before building it. And I think you'd end up wanting a much bigger head tube lug than shown.