BT can't know what the user needs. Users' circumstances vary. There are such things as families. I might be in the middle of watching a streaming video on Netflix or Amazon (can't even download from Netflix to save money by doing so when it's cheap rate with AA's 'units' tariff, or to allow later viewing when offline) and then Mrs Weaver finds everything has gone frustratingly sluggish while she's trying to get some work done on the web. Or Janet unknowingly starts a YouTube video and mucks up my streaming. And then one of us either is confused and starts debugging, or one of us realises the cause and then curses the other.
And we don't even have any kids.
BT needs to be aware that the requirement is for multiple users to be able to get reasonable small things done. Routers in future need to be able to ring-fence bandwidth, either temporarily of permanently, for certain services and protocols. Users ought to be able to pay (a lot, even) more per month for additional service levels. This can be done effectively, by buying multiple physical connections.
From A & A (AAISP), for example, an additional ADSL2+ line costs £12 per month, more if you, like me, choose BTW “premium” option priority traffic, when using the units-based tariff. FTTC or FTTP links cost more. Other services have totally different pricing models.
BT really ought to guarantee where reasonably possible that users can pay them for multiple physical links or higher grades of service by some other magic, because there's no one-size-fits-all.
And I haven't even thought about very small local businesses with a few employees, which desperately need various grades of upstream bandwidth. Tiny businesses round here can't afford thousands of pounds per month for an unimpressive Ethernet WAN pipe.