---Don't see expensive fat phones being popular. Attachable modules, attached directly or apart via wireless. You could do more than a camera attachment with such a system. Another idea, reflector lens design like that used in telescopes to get around the physics problem.
You are indeed CORRECT that very fat smartphones will NOT be popular! As I stated earlier, some basic research being done by certain manufacturers have indicated that 15mm (1.5 cm or a little over half an inch) is about the phone thickness limit for MOST consumers.
In terms of the focal plane issue with larger sensor requiring a greater distance between first lens element and sensor plane, this MAY BECOME A NON-ISSUE IN THE NEXT TWO YEARS once aspherical and/or compound-curve microlenses (i.e. Sony HyperHAD-like) get attached to the top of ALL photosites on an image sensor. These tiny microlenses WILL be able to reduce the number of lens elements required by allowing the extended REFRACTION (i.e. bending) of light to a more central focal point on each photosite! This super-saturates the photosites with more light rays giving a BRIGHTER image pixel that physically comes from a highly-specific area on the outer lens element. Compound curvature microlenses allows light coming from a highly specific 2D-XY area of the first lens element to be gathered and directed to a specific area on the photon sensitive imaging plane.
This means that a 9mm super-smartphone will be able to have as a large as a Full Frame sensor once the Big Boy manufacturers get on with perfecting the mathematical formulas that require that EACH microlens have a SLIGHTLY DIFFERENT curvature depending upon where said photosite is located on the sensor itself. Those photosites near the outer edges of the sensor will have a DIFFERENT high-curvature aspherical microlens than the ones in the centre which will have a flatter hemispherical microlens.
It is the computation of these microlens curvature formulas AND the manufacturing techniques required to make variable-compound curve microlenses and then place them correctly in the RIGHT 2D-XY location on any given CMOS image sensor has been an issue which is just now becoming solvable!
This is WHY 2/3rds inch, micro-four-thirds and ASP-C will be the FIRST sensor types coming out on phones that are about 12 to 15 mm thick. Only thick phones can (for now!) handle the focal plane issue of a large image sensor! For business people and tech enthusiasts who were used to the old heavy and thick Blackberries and iPhones from 10 years ago, this will NOT be an issue! They will grow ACCUSTOMED to the thicker more-ruggedized phone, ESPECIALLY when they realize it has a longer-life battery, a 4K resolution OLED display, a Qualcomm Snapdragon 855 CPU with 12 gigabytes of System RAM and 256 to 512 gigabytes (or more!) of on-board flash memory storage AND a super-low-light capable APS-C image sensor!
What's NOT to like about that!