It's not strictly speaking BTs job to upgrade everybody to faster broadband, they just react to market conditions and if nobody else is threatening them, then there's nothing to react to. Any provider could at any point in the past decade have gone flat-out on putting their own network together, they just didn't.
There are legitimate complaints about BT/Openreach, they were far too slow to realise FTTP was how things had to be, but we also have house builders who put absolutely no thought into Internet provision in new properties when FTTP would have involved no additional cost to them, and planning laws that didn't mean they had to take notice. It was quite laughable when companies like Sky who had built no network of their own and weren't offering any services on Openreach FTTP were complaining about BT holding the country back, that's just politicking at that point.
A lot of the issues BT have now are due to their heavily regulated status - I'm sure they'd like to withdraw ADSL, FTTC and G.fast from sale where FTTP exists so that they get a return on their investment quicker, but they aren't allowed to do that yet. While I'm ranting - Virgin Media Gig1 has an upload speed of 52Mbps and new areas are converted from fibre to DOCSIS at the outside of people's houses so some poor quality cable modem can still function, which is frankly a joke.
I think the best approach to this would have been duct networks owned by local authorities that all providers can rent access to, but who was going to advocate for public spending a decade ago.