The Rotax V6 has been 'coming' for about as long as some of the diesels. I wouldn't be at all surprised if we see a production Zoche diesel before a production V220 or V300T.
I think you'll find that the V220/300 has two overhead camshafts - twice as many as strictly necessary - more friction, more complexity, more weight.
Pop a FADEC on a Lyc and you'll see equivalent performance, multi-fuel capability etc as a Rotax without the weight, cooling penalty or complexity that comes as standard with the V300T
re: fuel cost, I agree that you probably can fly a 100hp aircraft for £1600 a year, but it's not the sole domain of the Rotax. You could probably come close with 1940's technology in an O200 or O235. On a PFA machine, it could be possible to convert to MOGAS too. Comparing it to a 180hp flying machine is pointless.
oh, and I could find at least 7 pilots in the UK that all fly around everywhere fully rich. 40L/h is scandalous!