Saying AC hasnt improved at all is quite unfair IMO. Compared to what AC was when it first left Early Access in late 2014, the current game in 2016 is fairly improved to the point it's fun to race these days. Perhaps it might not be as good as it's competitors, nevertheless, it had improved. I remember the AI used to push me out of the road at the first corner in every race, it rarely happens now. The AI used to be a push over in EA, now, they could put out a decent fight to a certain degree while able to pit now than ran out of fuel in the middle of the race. There are also other improvements like better driving physics, sound improvements, enabled 64bit support for better performance and some really subtle graphical improvements along with quite a fair bit of new free and paid content. I dont think they made any ground breaking changes to the game yet, there is still a constant improvement to make the game better. Kunos definitely isnt sitting there doing absolutely nothing for the last year and half.
The DLCs they released so far are reasonably priced while it is quite well packed with content. If they didnt release DLCs, how would they support the game for free?
Weather and day/night cycle is literally impossible at this stage as the engine only supports single light source where a major engine core overhaul is needed to support these features. Heck, even the cars head lights doesnt emit any light. I think it is extremely unlikely to see rain or day/night in AC at this point. These features are crucial in a racing game but I think it is something that more hardcore racing fans wants than casual fans where I think they rather focus their effort in improving the physics and expand the content.
The main thing for racing game is still the driving model which Kunos nails it very well. No point having all the features while the driving is terrible as I wont even play it if the driving is dull. Considering this is their first major game and the game is built on their own engine(not ISI or UE4), I wouldnt mind these missing features. However, AC2 definitely need good AI and day/night cycle with rain.
One thing is, I am really unsure how are they going to deliver a dozen of more Porsche cars considering their studio only has 30+ staff while my main problem with the game now is, the game really need more tracks.