I don't know how much aluminium is in the local loop in the UK - I suspect its not much. Copper was being installed a long time before the 1960's and afterwards. Aluminium was only used for a few short years - yes, better if the GPO/Government hadn't done so but it was fit for the purpose it was originally designed to do which was carry voice calls.
I don't think even BT know how much aluminium is in the local loop, there is no data available to show the exact number of lines. All I know is that there are many users on many forums that have been told by their engineer that their line is aluminium, or have seen for themselves when BT's contractors expose a cable bundle and leave it exposed a few days. Sure, people are more likely to post if they are having problems vs not having problems, but it's wrong to say you suspect it's not much, as you (or I) have absolutely no idea how much aluminium there is.
Hybrid copper/fibre systems like G.fast are going to be the dominant method providing high speed internet access around the world for decades..
In the UK, yes. In the rest of the developed countries, no. FTTP is much more popular when you look at other large countries rollout plans. Any technology involving sending RF signals down different lengths of aluminium/copper is going to suffer from many issues. These issues include the length of the cable, whether it's aluminium or copper, whether there are any bad joints, the cable thickness, local RF noise, electronic devices (from industry), rain getting into ducts/joints, and many more.
Xdsl will always have an 'upto' catch - sure some will get the max speed, while others on a aluminium/thin copper line, next to major industry producing RF noise etc, will struggle to reach the headline speeds.
Another reason why the level of FTTP differs - apart from things like government subsidy/regulation - is that the technology has improved that its now possible to use copper to deliver higher speed services which wasn't an option in the past.
Higher speeds on average, sure. There will still be users on G.fast too far from the remote node to attain sync. There will still be those who suffer from crosstalk. There will still be aluminium lines attenuating the signal. Solar radiation, rain etc will still drop speeds/produce errors.
FTTH is simply the wisest choice for the future. Significantly more reliable, significantly faster, easily upgradable compared to G.fast (by switching the terminating equipment) and not subject to distance, the weather, or RF interference.
There is a reason FTTP is being rolled out to a large % of the population in many countries instead of G.fast.
Of course BT prefer G.Fast - simply because they get to re-use their rusty old local loop, which is a major asset to them.