I'll try to answer your questions as best as I can.
For starters, most people today who dedicate themselves to martial arts fully today, don't train Wing Chun. That's a fact. You are not discerning people who dedicate themselves fully to MA in this day and age, to martial artists that dedicated themselves to Wing Chun solely and completely.
Mercenaries and soldiers don't need to be skilled, but the ones that want to live, do. Also, it was an important part of the culture that those who were considered Gentlemen amongst Chinese society always carried a sword and were well instructed in forms of combat from very young ages.
Wing Chun might have been designed to be learnt quickly, but again, it failed. You can't take a martial art that doesn't have sparring, and whose training is based solely on repetition, and claim that anyone can use it. The reason there aren't many skilled practitioners of Wing Chun in these modern times compared to before is simply that there aren't as many people who are willing to dedicate as much time to Wing Chun as old practitioners did.
Even the "average joes" before had jobs, but those jobs consisted of manual labour which in itself, is training. Contrast that to today, a large portion of the population's job involves sitting down and ruining your posture by staring at a screen for 8 hours a day, while eating junky and processed food crammed with unknown hormones and obscene amounts of artificial ingredients.
Lastly, Wing Chun, is considerably more technical than many martial arts out there. Its main offensive tool, the train track punch is a very weak strike - when you compare it to the boxer's cross, or the karatekas reverse punch it pales in comparison. Yet, Wing Chun is a developed system that is based on using this punch in quick succession to cause damage. What other martial art can you think of that bases its entire curriculum on an objectively weak punch?