The Swift Sport and Demio 1.5 both have better power-to-weight ratios than the petrol Cruzes. The Swift by quite a margin too, it has similar power to the Cruze with approximately 300 kg less bulk. Just an FYI.
I need to remind you that kei-cars are a very small part of (Premium) GT5, greatly outnumbered by sports coupe, GTs, hot hatches and supercars. And, yes, most of these kei-cars double as sports cars and are fun to drive for some of us. Why does it bother you that a small selection of a certain type of car that you don't like is in the game? I'm not bothered that there's muscle cars or pick-up trucks in the game that I don't care to drive. Muscle cars and pick-ups are no better suited to the track than kei-cars. But I'm happy that they (pick-ups) are in the game, despite having no intention to drive or race them, because others want them. Why can't you do the same?
And let's look at the (premium) kei-cars in GT5:
Suzuki Cappuccino (x2): Sports Car
Autozam AZ-1: Sports Car
Daihatsu Copen: Roadster, Sporty
Daihatsu OFC-1 Concept: Sports Car Concept
Mitsubishi i-Miev: Electric car, notable car
Suzuki Cervo SR: Economy hatch
So there's only 2 non-sports car kei-cars. 2! Less than one percent of premium GT5 cars.
Personally, I wish for two more:
Honda Beat: Sports Car
Alto Works Suzuki Sport Limited: Kei-car equivalent of 4WD hot hatch, fun (don't tell me otherwise, I have a blast with it every time around Tsukuba), GT mainstay
Clearly, GT doesn't have a kei-car problem, and you'd have to be very picky and intolerant to think it does.
Again, the point is not about those small cars. The cars you listed are "awesome small cars" and not in the category of an undesirable one, for example a life step van. Something you know it's awful and that at the same time there will be absolutely 0 fun in using in a videogame (because it isn't strictly related); that you would regret or feel bad about needing to buy in real life and that in-game is the most boring experience ever. Coincidentally there are lots in GT5.
Lets say there's this finite resource called time, that keeps moving on and can't be stopped. At the same time there's a deadline to be met. Not only that, there's a second issue called money, that doesn't allow you to have infinite employees traveling around the world modelling cars.
With that said, PD needs to choose what cars to get into the game, because they simply can't add them all. It's about preferences. It's impossible to pick "both" as an answer, as analog somehow answered.
Sure you can balance the list a bit, but there will be a point in which adding an undesirable car means not modelling a super car, because of finite resources.
So, as far as we know the only source of car preferences of actual customers and that's publicly available are wishlists. There's nothing more that has a 1:1 relation, or that comes close and without speculation and theories.
Lets think for a second. Based on those and on what the forums have been saying for years, which one of these 2 choices will players prefer making it into premium status: a honda life step van, or a bugatti veyron?
I'm not comparing the mazda miata against it, because that's desirable car to have too yet it is small and somewhat underpowered, or against the daihatsu midget, because that's a fun car to "drive" in GT5 (though not an original idea because it was featured in best motoring) which is the second criteria and thus is desirable in-game nonetheless.
That's why I say "econo-cars" don't have a place in GT6. In the end adding a micra means not adding the br-z