I think there’s another way to think of this, and that’s in terms of reliability and portability.
A breadboard is very flexible, but you don’t want to drag a breadboard with a bunch of hanging wires and other dangly bits to Maker’s Faire, or to a job site.
The next step up is the ability to solder, wire-wrap, or so forth on top of a prototyping shield (or to re-task a shield or similar that is close enough). Now at least you’ve got a smaller form factor and you can pack it in a bag and get it to another location without a bunch of connections coming loose.
Past that, to reach any more component density – as well as, really, to fit nicely into a housing – you need to move towards pcbs.
I think, tho, there is a different middle-ground you’d get by having stable (aka soldered-up) sub-boards that connected with cables. That way you have the stability of total system to carry it around, but more flexibility in layout than you get with shields. In any case, as much as possible you go with modular components, components with more features and redundancy than you need, and keep the selectivity mostly in software. That seems to be how robotics people are increasingly moving.