He has had good games as a winger, I never denied that. Your theory about Ronaldo is interesting and naturally having a player like him make space makes life easier for everyone else - the same was the case for Liverpool last year.
However, Di Maria works much better centrally as he can use those dribbling skills up against midfielders or centre backs and still has options to pass. On the wings he will always be doubled up upon and will almost always be stopped in his tracks as the shut out all space. In the middle, he has passing options around him which enable him to either do a 1-2 or to use the passing options to beat a man. Either way, him playing on the wing vs. Chelsea was never going to be insanely successful as they have some of the best fullbacks in the league and have wingers who do a stupid amount of work defensively, so Di Maria was always going to be up against it.
I have no doubt that LVG bought him to play at CM, and it is not a coincidence that the best year of his career and when he had the largest impact was when he was able to dribble and pass through the middle. Being played out-wide is no excuse for his lacklustre performance, poor passing and for giving the ball away a lot yesterday though.
Just to add to that, there are more differences with a more central starting position as well. For me, the most important is that because he can pick up the ball much deeper he will far more often come up against a line of defense that's not in balance, and because he is able to bring the ball forward so quickly and effectively it causes all sorts of problems for the opposition who will generally have to reorganize on the fly as their first line of defense is bypassed.
Contrast that to when he's played out wide. In those situations he will generally receive the ball higher up on the pitch and further out wide, but more importantly, he will be receiving the ball in a situation where he's almost always coming up against a balanced defensive line. At best, a one on one against the full back who's very prepared for that situation, or even a wide midfielder / winger tracking back. In comparison, when he's played more centrally this season, he is usually getting the ball deeper, and dribbles / outruns the opposition's midfield with it. That means their defenders have to make a choice of who should step forward to press him, and who should stand off. This immediately causes unease for the defense as a whole, and much more so than compared to when an orthodox winger receives it out wide.
As someone said, this could of course be applied to all wingers. Well yes, in theory you'd want players who have pace and can dribble centrally, but the reason that it works especially well with Di Maria is because he is tenacious and has a very high work rate, and is good at picking up the ball in the right positions. Call it a combination of tactical awareness and physical capability that is necessary to play centrally. As a result, the positives outweigh the negatives that you usually would get from putting a typical winger more centrally.
If we had one or more midfielders other than Di Maria who had close to his ability of driving with the ball in midfield, I would be all for having him out wide. But because we don't, we lose a dimension in our attacking play by placing him there.
So I just want to make it clear that I am not saying Angel Di Maria is any less of a footballer if he plays out wide. But because of the composition of our squad, his impact will be lesser when he does. Ideally we should have had two of him, one to play out wide and one to play more centrally.
I'm also a hundred percent certain that LvG knows all of this, but has decided to play him out wide the past two games anyways for one reason or another. Yesterday I think the main reason was that he wanted to have Fellaini in that slightly left central midfielder role to do a job on Fabregas, and it worked quite well.