FSN is, like I said, like a mountain. Fate is the base and ubw and hf twin peaks. The whole thing only works when you put it all together. Any one part missing and the pay off isn't there. Like Ilya, you learn her character in fate and when UBW rolls around and that happens it's not just that Gilgamesh killed someone it's that he killed a nice person, a terribly pained person at that. So of course you want to make Goldie bleed from the eyes. But that further stretches into hf. Couldn't save Ilya before but now you can so the moments intensity is through the roof. Point being, the idea of leaving anything out makes a flaw product but here it takes the steam out of the train.
Look, that is entirely correct in reference to the VN. Each route builds on the last, and relies on it. The story exists as a connected whole, not three seperate entities.
However, there is simply no way to adapt a Visual Novel that retains that connection, not least because it relies on you being directly involved in making the decisions, as opposed to watching from a distance as you do with an anime. Therefore, the only option is to treat each route as an individual entity, and try to make sure that as much as possible of the principle of the VN is carried across anyway. It's difficult to do, yes, which is part of why the Deen anime sucked, but there is no way that you can say it is certain to fail without even
seeing the anime.
I agree that there will possibly be some issues with making the UBW anime work for people coming from Zero, but that would have been just as true if they'd started with Fate, because people would treat it as a single, stand-alone story, even if they were intending to adapt the other two routes. Indeed, I suspect one of the biggest problems will be that people have watched the stories in the "wrong order" (i.e. Zero first), meaning that things that you're not meant to know when playing UBW are known. For example, the UBW ending comes across as rather unsatisfactory once you know about Sakura, and I'm not sure how they'll resolve that problem.
However, with regards to Fate, I don't think that there is anything essential in it that cannot be added into the UBW anime perfectly fine. Yes, there will be some things that are incomplete or which make less sense because of the lack of context, and some characters will be under-utilised, but I don't see how that is going to result in an adaptation that doesn't work. UBW as a story can mostly stand on its own two feet, as can HF. Further, even if that were not true, those of us who have played the VN or, even, seen the original Fate anime, will be aware of most of the stuff that is needed from Fate
anyway.
You may be right that the anime will turn out bad, and it is certainly difficult to adapt a VN into a good anime (especially a route that assumes previous knowledge). However, I see no reason to say that it is
impossible to do so effectively, or that the anime is doomed to be bad from the beginning. I see no reason to assume that Ufotable won't do a decent job of it.