No, not naive. Very sophisticated cost models used in aerospace industry to predict the cost of spacecraft. They look at al the factors one would expect. and you know what? In the end it comes down to reasonably constant dolor per pound. This of course assumes a constant level of technology and the engineers always try and put the pace and weight to the best use. The result is very surprising at first but it "just works out"
Even if we assume that results from aircraft engineering are directly transferrable to computer tech, there is a problem with "constant level of technology" part. The thing is, Apple still uses high-performance components in their very thin laptops (there is no laptop on the market with a faster CPU for example, save for behemoths that use desktop-class CPUs). And providing adequate cooling and engineering around these components in a thin and light chassis is simply more expensive.
Let me give you a practical example: Apple uses AMD Polaris 11 GPUs for the 15" MBP. These GPUs are not the fastest of the bunch, but they deliver very reasonable performance to power ratio (which is more or less the same as best Nvidia offerings). There are few things which make Polaris 11 in the MBP special though. First of all, they use a full chip (with 16 out of 16) cores enabled, while everybody else uses versions of the chip with 14 cores enabled — the full chip is obviously more expensive. Second, they have additional manufacturing steps applied to the chips (die thinning) which makes them more compact and more energy efficient. This is quite advanced, expensive stuff right here. If wouldn't be necessary if the laptop was bigger of course, but that wouldn't make the laptop any faster, since the GPU without all those extra expenses would simply run hotter.
Another examples include low-power RAM modules which are more expensive then the desktop DDR4 variant, symmetrical Thunderbolt3 ports, which are more expensive than the normal USB-A or HDMI or Magsafe, the keyboard which uses more complex mechanism compared to the traditional etc. Regardless of what kind of criticism one has for the new MBP, it is a basic fact that tech it uses is simply more expensive than found in many other laptops. Ever wondered why most laptops until not long ago uses blue-coloured USB 3.0 ports vs. "regular" USB-2.0 ports? Because USB-3.0 implementation required extra chips and was more expensive overall — so manufacturers use these kind of hacks to cut cots. Its not something that Apple does.
Lets say Apple management gave their engineers a 1" think "budget". They would put the ports back, they might put in a faster CPU and a lot more memory and a much larger battery. All in all they would stuff the space with about the same kind of parts in about the same proportions.
Again, this is not an airplane. As mentioned before, they already use fastest CPUs available, can't go faster than that. battery could potentially be increased by 30%, true. RAM is a more complicated problem since going more then 16GB would require DDR4, which would probbaly more then negate that 30% battery increase (based on independent tests, 32GB of DDR4 uses around 12 Watt alone — thats the same as the entire 2017 MP on idle). Same with faster GPUs — using a faster one means need for larger cooling, which means that the battery size can't be maximised. Etc. etc. In fact, when you look at todays industry, you'll see that manufacturers are going the opposite way: they are downgrading the CPUs to lower power levels and using large batteries to deliver long runtimes. Which makes sense, since most computer workflows are based on bursts of CPU activity. Unless you need sustained performance, in which case the MBP is practically unbeatable.