Imagine, if you will, a car company updating the interior of their cars every year starting the year 2000, but not updating the engine, transmission, body style, safety features, built-in software, collision ratings, etc. That car would NOT be worth it to anyone because it'd be using the same tech, body, software, safety features, engine, transmission, etc. as a car from the early 2000s, and therefore would not be a wise purchase, not to mention it's literally unsafe to drive and probably costs more to maintain than a brand-new car. Who would buy a car listed as a 2023 model with a brand new interior, but with internals that are 23 years out of date?
If JointedRail were to specifically update ONLY the cabs of their GP7/9/18, likely nobody would buy them. Very few Trainzers actually spend meaningful amounts of time driving trains in the cab view, and just about everyone wants an upgrade of the model itself. In fact, custom cabs are the least of my concern when purchasing a new model. If I saw they updated the cab (but not the locomotive itself), I wouldn't buy it. When JR upgrades or updates a model, the cab is updated along with it.
Even then, the model and cab on the GP7/9/18 is still perfectly fine. It looks just as good as it did when it was first introduced, and is one of the best cabs made for Trainz, as per JointedRail quality.
All to say, have fun spending $14.99 on a seven year old model with a slightly better cab