I agree with your sentiment, but surely such a list would be more powerful than using the official dark angels? This is my point. Using a different list to get access to a different FOC chart to allow that themed army yet still allowing you to use the miniatures you've already bought and painted for a different army just adds variety.
Swapping for other reasons as you say can get tricky with the understanding, especially if the same models can be used in the original list but with less powerful rules. Counts as should be about the models not the rules right, otherwise you are just wanting to win?
Now, seperate to this, the part I wanted justified, is why an opponent would refuse to play a red terminator counts as deathwing list, yet would play a deathwing list. The game is about lists and rules, and the game would be no different. Both armies would be fully painted which surely is good for the visual side.
Whilst I'd happily play my BA terminators as BA with a tweak to just allow them as troops as long as I stuck to First Company troops and equipment, I would expect far more resistance to this, than using a deathwing list.
I bring this up because it was specifically stated by a few posters that they would not want to face a red deathwing list because it would ruin their game, and I still struggle to understand exactly why the colour makes a difference, when it can't just be fluff, as the rules already do not match the fluff in most cases, and red DA is no different to orange BA because I over highlighted - once they are no longer in keeping with one's view of what a blood angel, or dark angel should look like, it is already the wrong colour. Why is a custom chapter more acceptable than a regular one using a different list when all is wysiwyg but the shade of paint?