Dave that's almost exactly what is happening, the installation of FTTC means you've got a network of fibre spreading out from the head end exchange, when G.Fast starts going in the network would spread out further, I think of it like roots from a tree, trouble is some tree's take a very long time to grow
As we already know this network of fibre can be used to supply FTTP
I'm not sure we'll ever see a law requiring fibre for new builds, but it would be very welcomed if councils pushed for it on new estates, on insist on it where BTor is willing to do it for free. One thing they could do is make it a legal requirement to install proper ducting, this could be implemented as part of the Building Regulations, this wouldn't impact builders costs as ducting is relatively cheap, but would make future installs of FTTP much quicker and easier. There's clearly a lot that can be done to help things along, but it's getting the people who have that power to implement, most of them probably haven't got a clue about FTTP, FTTC or ADSL and it's differences.
There's a large estate being planned to replace Manston Airport and I've emailed the developer a couple of time now pointing out the benefits of full fibre, and last week pointing at that BTor will to do it for free on the size build they are talking about. I've also in the past made comments on the planning portal for another local large development saying they should insist on full fibre.