Benefits of a mirrorless FF?
+ all IQ benefits of FF sensor = technically better images in any shooting situation (except Macro); more appealing pictures when shallow DOF is advantageous (e.g. often in portraiture)
+ significantly smaller and lighter than FF DSLR = easier to take along and to travel = more pictures at interesting places
+ significantly smaller and lighter than FF DSLR = less conspicuos = more and better images in any "non-staged" shooting situation
+ significantly cheaper to produce and service than any DSLR = = more profit for manufacturer and/or lower cost to customers
+ no mirror slap, less or no vibration = more sharp keepers, especially in tough shooting conditions/low light
+ no mirror = shorter / no viewfinder blackout possible
+ silent operation at full speed possible (with silent shutter; unfortunately not A7R) = huge advantage in noise-sensitive shooting conditions
+ no hard-to-clean oil-debris-splatter on sensor from flapping mirror mechanism (-> Nikon D600)
+ EVF better than any OVF (soon to come) - all shooting relevant information overlayed, camera can stay on eye and image displayed exactly as it will be captured = better images, more often capture "at decisive moment"
+ shorter flange back = use of almost any previous lens via adapter possible (but not always with good results)
+ AF performance inclduing tracking moving subjects will surpass capabilities of today's best DSLRs (soon), because no mirror in lightpath = more keeper action shots
- new lenses needed for optimal image quality and system performance, with less bulk and weight
- operation with 600/4 lens still requires sturdy tripod for optimal results
Agree with most of the above but what you refer to as "coming soon" is a good bit of conjecture - but who knows, technology changes faster than we expect. However, absence of exceptional AF, the limitations of EVF and a lack of dedicated ML lenses are a potential deal breaker for most (including me). For the moment, DSLRs are still more functional.
Also, I'm not convinced that when all this is made available, the price of the FF mirrorless will remain the same.