I think the discussion has to this point revolved around answering the risk/benefit question in that people are saying, implicitly if not explicitly, that once someone knows they system to a level of adequacy and proficiency they should be able to make that call themselves. Until they know and understand enough of it people really can’t (to a sliding level of degree) make that call.
And when one is teaching, you can only teach what you learned and of course anyone with any competence should be able to provide good reasons for doing it they way they do and not some other way. One should also have a completish picture and know how each thing integrates with other elements of the art. If a student improvises something in sui lim tao they might inadvertently stuff up something in bui ji they haven’t even come close to learning yet. Doing such improvisation is an enormous risk. Without an adequately wholistic picture of as much of the art as possible one just can’t know all of the risks or all of the benefits in doing a given element of the system one way or another.
The footwork is an example. It takes a lot of training to be able to do wing chun footwork properly. You won’t have even learned all of the elements of a basic step, lets say, cutting a diagonal line on someone, until you know at least the first three forms. Then to make it actually work takes a huge amount of training on top of that; and some of the most useful footwork for perusing a fast retreating opponent is from the knife form. So, what do we encourage students to do? Just do as they are told and train so that one day it will work for them provided they put in the training? Or do we encourage them to improvise and do what works for them? If we do the latter they will decide: “this doesn’t work, the risk is I get punched or kicked. I’ll do it this other way instead, that stops me getting punched or kicked”. Chances are too that whatever way they figure out themselves to do it will have inherent risks or very low benefit compared to risk that they don’t even know about. Is that not all dealing with a risk to benefit ratio?
And as far as top masters never having learned the knife form from Yip Man: I don’t know who learned what from Yip Man but I suspect there are plenty getting around who are top masters for little other reason than that they learned from Yip Man. All of the forms in wing chun are mutually supportive, in both directions, including the knife form. That isn’t to say you can’t have good wing chun without it, all I’m saying is that you can’t have a complete picture and contextualization of everything without it; example, the knife form footwork is great for when someone retreats very fast, better than the stepping from chum kue, if you don’t have the knife form you wouldn’t know that or be able to do it.