Why would you assume that Tesla plans to reduce prices of the Model 3 by using "slight tweaks or software limiters"? That makes no sense, Tesla is on a mission to reduce the cost of EV's in fundamental ways by making the cars easier to manufacture and with lower assembly costs and lower parts and raw materials costs. This requires optimizing the engineering of both the production lines and the car. The goal will be to have the same functionality, the same reliability and longevity, the same size, at a significantly lower cost, thus driving the adoption of EV's into ever larger market segments.
People over-estimate the savings available with a smaller model, making the car smaller on the outside and more cramped on the inside. Sure, raw materials can go down by 15% or so, but raw materials are a small fraction of the total cost to produce. You still have four wheels, and you still have to install, inflate and balance four tires and wheels. You still have four seats, carpeting, a brake and an accelerator, sun visors, door handles, air bags, parking brake, etc. etc. etc. Costs do not even come close to scaling with size. The Model 3 is at a sweet spot in so many ways even if a smaller car has some advantages in the middle of European cities. Making a small city car will happen, the question is whether now is the right time or not. The Model 3 is a greatly under-utilized asset of Tesla and it costs too much to build. Tesla is going to fix that, and it will dramatically expand the addressable market and the profits from Model 3.
Aerodynamically, it's difficult to beat the Model 3 by a lot as it already has a pretty small frontal area and the Cd of a smaller car will actually be considerably worse. People who understand aerodynamics get this (you have to multiply the frontal area by the Cd). The net effect is not a wash, but it's closer than you would think. So, to get the same range, you still need 90% of the battery. The battery of a city car will be more than 10% smaller, but mostly due to a shorter range and gains in efficiency from technological advancements, the gains in efficiency due to the smaller, lighter form factor of a city car over a re-vamped Model 3 will be relatively small. People over-estimate the actual efficiency gains from making an electric car smaller than the Model 3.
A revamped Model 3 will be lighter and just as aerodynamic as the current Model 3, without losing value by scaling it into an economy segment. The battery will be able to have fewer kWh because of that (and efficiency gains from motor/drivetrain optimizations). It can sell for a lot less than the current Model 3 and have better margins. That is simply not possible with a city car unless you start compromising on quality, longevity, efficiency,and, to some degree, safety. Tesla naturally doesn't want to go there until it's necessary to continue expanding market share.
I'm not saying Tesla will not announce a city car at the same time, but they need the Model 3 to be more compelling from a price perspective so the city car does not steal the show with lower margins. There is room for both, and the Model 3 is the ideal platform to be the next stage of increasing volumes worldwide. The reason for this has to do with the differences between gas and electric platforms. Gas cars have more compelling economic advantages when scaling smaller than electric cars do. I won't go into the physics and the exact economic reasons why, but the difference is significant enough that it will change the adoption rate of the different form factors to a slightly bigger car. There is still a place for a small city car, it's just not as compelling as with a gas platform and the adoption of EV's will benefit greatly with an affordable and desirable car like a cheaper Model 3. This will benefit Tesla.
People who think they know better than Elon Musk, generally don't.