I've spoken to a couple of people recently who are close to Bethel in the US and UK. They have both provided anecdotal "evidence" that jw.org gets lots of request for bible studies, especially from areas where there are few witnesses.
What the facts are about this I don't know but bearing in mind you only need a handful of requests from each country each day to get some fairly impressive figures then it could be true.
I really don't believe the carts are effective. I have seen the carts in lots of situations now and it's simply a good way of counting time. I have watched the carts in areas like Oxford Circus and several of the London main line stations and never ONCE seen anyone come up and take literature.
Interestingly enough, the carts do make effective use of literature. If no one takes anything then you don't need to restock much. If carts were used instead of the D2D work then the literature requirements would drop massively.
It would be very interesting to see some analysis of the publications over the past 10 years and see what the stats are of the mentions of D2D ministry v construction & donating. I know it's probably not possible to do but I would suspect there has been more and more mention of donating and construction support than people realise.
I don't think the D2D work is going away any time soon. It's a great distraction for the masses. The other thing is it's a great barometer for judging "faith". The WTS has always equated lack of zeal for the ministry with spiritual weakness. They've spent years banging on about the D2D activity being done by the 1st century Christian. It's also considered a command from Jesus therefore they would need to find a way of mitigating any U-Turn on D2D work. Regular field service is a key aspect of the Witness profile and there is not enough cart work to keep all visibly occupied.
What I think we are seeing, however, is an increase in the discussion of construction projects, especially the numbers. The goal is to keep the numbers looking large so people are prompted to donate and give their free labour.