Grabula,
You post good questions. They say “90% of all fights go to the ground”, but 100% of them start standing up. A Wing Chun person ideally trains to be a superior stand-up fighter. If your training limits you to focus only on techniques, then you will think there are deficiencies in the Wing Chun system. Wing Chun is a complete system. If one is trained to think more openly, you will find that mastery of any martial art gives you a “martial mind”. At this level, you will find that you can grasp some value from looking at an application of a technique and get it in half the time! Why? Because at the true higher levels, it is the approach to the training that matters more than just the toolset of techniques. Just becomes another technique you can apply your principles and attiribute training to.
Interestingly, when I had a conversation with a very good BJJ person, he used words that reflected a nature about the art more than just a list of techniques. He talked about balance, coordination, timing, sensitivity, and relaxation. About flowing with the opponent. Of course he also mentioned about chin down, and open limbs etc. Not that I became an expert at his art at that moment, but within minutes I could gain some significant level of understanding of technique application.
We should be open to learning how to fall, experiencing grappling, experience kicking ranges and ground fighting. Those are not typically part of the Yip Man syllabus, but can always be a part of “your Kung Fu”. Some have mentioned that they learn ground fighting in their formal Wing Chun training. The Wreslters and the BJJ people feel “at home” on the ground. In the same regard, most Wing Chun people train to feel “at home” standing up with the same purpose - being proficient at what they do best.
We can all argue whether ground fighting is a formal part of the system or not. The Wing Chun police are not going to arrest a Sifu for teaching ground fighting or developing some useful techniques for it using Wing Chun concepts. If you have the time, effort and the money and ground fighting is important to you, then train it! Personally, there is a wealth of Kung Fu to learn in the stand up arena of the Wing Chun system.
Good luck with your Kung Fu!