This is simply not true in my experience. My Mk7 TDi 150 is getting about 4mpg MORE than my previous Mk6 TDi 140. Road tax is 85% cheaper (just 20 quid now) and insurance was 9% less. Running costs have been the lowest of any modern car I've owned in the last decade. I'm also glad I dropped down from 18's to 17 inch alloys as the tyres are about 40 - 50% cheaper and don't break my spine anymore.
I've just had the best fuel range this past 2 weeks or so, with a range of 655 miles on a 49 litre fill. True fuel economy calculated at 60.74 mpg. The best calculated figure (not the crap MFD) I ever had on my Mk6 was just over 56 mpg and that's on the same commuting route, driving style, etc. This week I managed 640 miles on another full tank @ 49 litre fill, so it's no fluke.
People forget that the Mk6 MFD mpg meter was hopelessly inaccurate and over-optimistic, mine regularly displayed 10 mpg more than the true figure before it was traded in. It once displayed 74.5 mpg on a 32 mile return leg home - that's several more than the manufacturers figure and a statistical improbability (I calculated the true mpg on a full tank was actually 56 mpg).
Also a huge plus factor for me is that after 16,000 miles, I've made exactly ZERO warranty claims on my Mk7. My old Mk6 went back 6 times to the dealer for various quality & fit and finish problems that plagued it after just the first 3 months of ownership from new. I've had none of wiper or suspension issues that some have reported with their Mk7's.