Of course, TK spent far less time and money developing FE and it also promised less. It's the difference between buying a Ford Focus and finding it reliable and comfortable and buying a Lexus and finding broken functions, poor reliability, bad mileage, etc. You expect more from a higher-end product, not the same as the lower-end with just a shinier coat of paint (or graphics and FM/DM in this case?)
TK knew from the start modders would fill in the blanks by making his engine amenable to easy modding and based on the history of his previous sims, especially the heavily-modded EAW (even though that sim came out of the box with a ton of features, planes, and a nice big map of Western Europe). However, while modding does create longevity for your original product, it serves as a disincentive to buying expansions thanks to the "why should I pay $XX for those planes and terrain if I have this free 3rd party mod for it?" attitude. If you release a full-blown sequel you risk fracturing the modding community and it needs to be done after enough time has passed.
If neoqb has decided now to start allowing it, after setting themselves up as basically a sim structure that they would micro-charge for extras like planes, it's almost a tacit admission that their business model was badly flawed.