So, I was thinking this morning-- and just posted Dpr
In the film days, we would buy into a system, based on both camera bodies and lenses. You had to weigh the ergonomics, features, and production cycle (sometimes 8 years) of the body AND the lens lineup-- which lenses fit your style best. (assuming we are ignoring factors like flash capability).
This could mean rangefinder, SLR, or even medium format versions of both.
Then, with the advent of DSLRs and the tremendous acceleration of camera body production cycles/obsolescence, the lens system became the prime consideration-- the technological leapfrogging of sensor and body combinations, and rapid depreciation of camera body value made the lenses the most stable, logical prime factor in choosing a system (leaving niche products out of the equation).
Im wondering now, with mirrorless leaving infancy and heading into childhood, and especially with full frame mirrorless, if we arent entering into a totally new age of equipment. Unless you have a very specific need, like high speed AF/FPS, there is no longer to choose a system at all.
I have the A7r, and have been using:
Canon EF lenses with full functionality
Olympus OM lenses
Leica and Voigtlander M mount lenses
Sony E mount lenses
Canon RF lenses
Canon FD lenses
Nikkor AiS lenses
Essentially, the A7/A7r (or any mirrorless full frame with EVF) allows one to cherry pick lenses from across almost every system, and I assume in the future, mate those lenses to any body that will accept the correct adapter. (with of course some limitations).
Granted, not EVERY lens functions to its full potential, but it seems as if there is suddenly a vast array of possibilities that were not there-- and freedom from systems.