What is the viability of stacking passengers in an airplane cabin?
I see several challenges with this design, both technological and not:


Making that design safe enough will be difficult, the structure you add in the cabin would need to be strong enough to maintain rigidity in a crash. That much rigidity would add weight, and would add loads to the structure 
The pod walls will make it hard to get by people in the single narrow aisle. In an open design you can squeeze by people in the aisle, in a walled design you'd have no place to go
This design would greatly limit carry-on storage to only what could fit under the seat in front of you
Parents with small children and families traveling together would have a hard time with this design as there's no more than 2 seats together, and limited space for sky cots
Access to a sick passenger would be restricted
Passengers with limited mobility would have a hard time getting into and out of these pods
Comfort is the biggest problem I see with this design. Anyone in one of those pods is spam in a can with limited visibility and movement. Personally I think it looks awful and I would go out of my way to avoid flying with an airline using it