The main cost is in digging. Not in what you put in the resulting hole.
As far as I can see, the biggest driver in laying cables today is to aim to never need to dig them up and replace them again ... if at all possible. And certainly not en-mass. This is the #1 priority.
Given what we know today about what might happen with data demands, it makes sense that you only consider putting fibre in ... providing you can get that fibre to work with your nearby infrastructure easily enough.
Result:
- Where there is no existing street infrastructure, put in fibre. Then use RFoG to hook that fibre into the existing core infrastructure.
- Where there is existing street infrastructure supporting coax, lay down more coax.
The same idea governs BT with new-builds. For a long time, new-builds should have been built with fibre (or at least ducts for the fibre) in the new streets, but they haven't always had fibre in the existing streets (back to the exchange) for them to connect into.
In the period of 2008-2013 (ish), it was probably right to build estates with copper plus empty fibre ducts, because there'd be little fibre spine to connect to. Now that has changed, and with a more ubiquitous spine in place, there is a better scope for BT to have swapped to a fibre-only design.