Ever considered that you can carry an A7rII (or any other mirrorless camera), a memory card and a stack of spare batteries, and still end up carrying far less weight and volume than a 7D with one battery?
Sure.
Battery life of A7Rii (according to cipa): 290
Cost of a Sony A7rII: 32.995,00 KR ($3888)
Cost of NP-FW50 battery: 795,00 KR ($94)
Cost of AC-PW20 charger: 1.395,00 KR ($164)
Battery life of 7D (according to cipa): 800 (I believe every 1/2 images using flash?)
Cost for keeping my 7D: 0
I don't know how many batteries are needed to get my personal "lasts for 2 weeks without charging", and how many chargers would be needed in order to make that setup viable, but it would add significant cost to an allready expensive upgrade.
I'd say that my original statement still holds well:
Could I work around having 1/2 or 1/10 of my current battery life? Sure. But that would be cost and/or inconvenience that would be have to be weighted in the pros and cons of camera upgrading.
Or that the lack of battery capacity in mirrorless cameras isn't due to them being mirrorless, but rather them being designed to be small, thus necessitating weak, small, batteries? Make an SLR that size and you'd also run into battery life issues.
I'd suggest that it is mainly the "mirrorlessness" that makes these cameras have low battery capacity.
My 7D have a "mirrorless" mode (Liveview). It has horrible battery life (using the same battery).
Canon offers smaller DSLRs (the 6D and the 100D) with what I believe to be decent battery life (small DSLRs can have decent battery life).
I believe that running a sensor continously "on" (feeding the low-latency AF and viewfinder), doing continous image processing (feeding the viewfinder) and powering an EVF/LCD, in sum, makes for significant power draw. Perhaps in time, this draw will be small enough so as to be insignificant. Certainly the current development in cellphone cameras and mirrorless system cameras got to help.
-h