Confusion and arguments among Yip Man's students comes from market position , not from the art its self . Some always want more , and thous who have more do not want anyone to take their position.
I don't think Yip Man changed core of his art , that is the same among most of his students (yes, some of them couldn't understand everything , but there will be people like that in every system) .
About need for constant changing , I believe that is necessary , our knowledge grows, are way of understanding things is changing, the way of fighting is constantly changing , art can give the answer on all that changes or not. Arts stuck in the past , stuck in a feeling that they have perfect set of concepts which will give them answer to any possible problem will have a problem . As much as concepts can be good if there is no work on their practical development , testing and adjusting to the most common threats of the time the art will be inefficient .
About laws of physics , I do not agree on a way you connected them to wing chun ( if i understood it wrong I apologize in advance) . It is true , laws of physics do not change and human body do not change , but I have a feeling you want to say Wing Chun is based on the laws of physics . It is not , everything we do , including wing chun is bounded by the laws of physics , but the art its self is not built on a foundation of modern physics ( I am sure that founders of the art didn't know even Newton's physics , about space, \time\energy\gravity\force vectors\ect, all of that are modern terms used by people of this time to explain what they are doing) . Wing Chun is based on understanding of the human body and fighting models of the time of its creation ( pretty limited). Wing Chun is a good art , most arts are based on efficiency , preservation of the energy , structure , economy of motion ...ect. Wing Chun is not better or worse than most of other arts. Straight line is not always the best course of action , not the most efficient one. I know for a fact that some arts that use wide circular motions use much less energy to execute these punches and they generate much stronger force ( Pi Gua for example ) . Some arts have very efficient angling approach which is extremely difficult to counter , very powerful strikes , also use center line theory , and all things numbered above (Baji) ... Wing Chun is good , but, like everything else , requires a lot work , is some cases much more work than other arts .