You are also assuming that everyone with decent ADSL in your rural area will upgrade to full fibre when it is available.
You'd be surprised. Especially over the past year, with increased WFH and remote lessons for kids and so on, having fast broadband (faster than FTTC) is more important than ever. My wife and I both work from home and when kids are using the internet as well, 80/20 can get pretty stretched. There's also the fact that most people will not get full FTTC speeds and various other factors can affect reliability and stability of the connection.
My colleagues in Southampton who live in the coverage area of Toob have all upgraded. At £25 a month for a 900 Mbps symmetrical connection, why wouldn't you?
My rural area, ADSL is probably 5Mbps at best, the exchange is too far away. Most with 'decent' ADSL are probably still likely to be slower than my current speed of 20/1Mbps. Like I say, 7 out of the 10 households in our area have opted to upgrade to FTTP as part of a CFP.
Myself and my partner WFH over the past year, she is a teacher doing online lessons, apart from our poor upload speeds we didn't have any issues (however, we don't have kids). I don't quite understand how your 80/20Mbps connection is being stretched though - in our offices, we have around 30 staff, we have a 70/18Mbps connection and we don't feel the need for any more bandwidth currently (we're uploading/downloading files, managing servers, Google Meets/Zoom, using the internet for our call centre system, etc).
The Toob service, I agree, but the price point is the same as what I pay for my current connection. It's a different value proposition compared with the Openreach 900Mbps service (£25 vs £60) - I personally can't justify spending £60 for a 900Mbps service (different story if it weas £25!).