I’m inclined to agree on all counts, but I also think we’re working with a less complete view of circumstances than you assert. By all accounts the BBC is not in a great financial situation, so the difference between 2K and 4K might matter.

Edit: I’m now pretty sure The mirror’s estimate of 4K conversion cost is much too high (maybe the mentioned sum covers a season, not an episode), and so my own calculus needs to be scaled down as well.