With these techniques - how complex a program can practically be created in 2018?

A lot of the example programs mentioned feel more like functions (stateless + deterministic mapping of a tuple of inputs to a single output).

In contrast, I think of a 'program' as having a longish lifecycle comprising a stream of inputs and outputs.

Relatedly, I'm not clear how well the approaches detailed here actually would scale with complexity - is this a meaningful building block in building more complex applications? Or is some higher-order framework needed that 'knows' how to apply this in a rich context?