They've had 18 months and have given us no major updates to the core game so far...
Sorry Simon, but that is pure nonsense.
In past 18 months we've got 2 major revision of the handling model, introduction of Seasonal Events for both A/B-Spec mode in either Races/Special Races/Time Trial and Drift Trial events, introduction of Remote Races for B-Spec, numerous functionality and option enhancements (especially in field of online racing options), new cars/tracks/gear/locations content, introduction of Online Collector Dealership, constantly improving appearance for standard models, etc.
You will never witness "major updates" to A/B-Spec races or Special Events races - if you somehow consider only those as "core game" (either they represent only one part of actual game, you can't really exclude Online capabilities and act like they do not exist). Why? Because they're tied to Trophies-system and you would compromise the logic and complete engine behind Trophies. And there are people that actually care about those as I heard (not myself, but unfortunately I know many that do).
If more A/B-Spec events would be added you'd mess the complete scheme-grid of the Trophy system and majority of Trophies are interconnected to completion of either percentage or some special part of the single-player part of the GT Life. And that is unfortunately not possible to do.
They expanded A/B Spec modes through Seasonal Events and they basically updated the complete "core game" with that. In fact, they've already expanded complete Seasonal Events
twice, because after one of the updates all *original* seasonal events were replaced with new "A/B Spec Challenges" after which the new batch of FR/FF/4WD/F1/Real Courses/Dream Cars, etc. challenges started to appear.
As far as UI is concerned, it falls under the subjective category. I really love the GT5 UI, I find it logical, easy to navigate and very intuitive. Loading times are the only real issue, but I can understand it because when you jump from GTLife menu to GT Auto game have to load completely new *mode*, with animated LOD of the car, animated mechanics, separate navigation, etc. When you return, same have to happen.
Also, when you're loading garage with 1000 vehicles or 2500 paints, RAM has to load 1000 thumbnails or 2500 paints, come on, be realistic, there is some data to be processed, it is not a 8GB RAM PC, it is 256MB hardware from 2005. Cut those hard-working guys from PD some slack for effort to present us with different concepts than just "click through simple still menu where nothing happens - done" and stop acting like you don't understand how it takes some resources to achieve something.