The Inflatable kite may well be close to its best design in current non-specialised format.. Stagnated - heavy word - but applied to anything which changes less than say 2% in 12-18 months - then yes...
Early eighties - Hang-gliding was in a rut - with High drag profile slow speed fun machines with a lot of limits...
Then
- 30% double skin came in (the Atlas - French design!!) then 60% up to 85+%
- Better fabrics - less stretch - tighter wing profiles
- floating keel pockets
- Floating cross bars - + variable wing geometry
- much more profiled battens... etc
- body bag harnesses


Result - top speed 3+ times greater whilst keeping bottom end speed and control (well most of it) - the envelope of the possible really expanded - at the time (in my 20's - jumped at the changes - loved the evolution).
For the next 10+ years it retained the generic top end shape as the must buy model.. bringing most of a shrinking market with it. Then a range of simpler return to the slower fun flying machines concept re-evolved. The market models split to appeal to an audience. The front runner designs pushed the club flier to their limits physically and skill wise (+ good bang 4 buck but expensive)....
The same changes in aerodynamics will start to be applied to the Inflatable kites - the aim to expand the wind range the speed and the lower the effort needed for endurance. I wonder what shapes could evolve if the C of G was moved back - with a possible bridal support on the center strut - so hybrid between foil and Inflatable kites.
The key to the current designs is the retained simplicity of fitting it all into a small rut sack to go home with. From 7-77+ yo can use it - it offers sophistication and is really crude (Think self-launch on a beach - think beach ball valve (some are better now

) - multi-purpose fabric (tents

etc). The main unique feature of a kite and 'IP' is its control bar - now how much more can you do with this???.
I think evolution is likely to create the specialist fit - for racing - for enduro - etc.
Look at how close (not) the sailing speed record boat is to a dinghy....
The market is till expanding, whilst it is the generic multi-purpose kite will dominate.
Off the shelf parts are cheap(er) - Given the choice of this against say adding some semi-rigid parts into the kite??? so I am ok with stagnant.


FWIW - I still have a lot of catching up to do to maximise the current design..
Cheers
AP