If HS2 is permanently curtailed to Old Oak Common, then people from Birmingham, Manchester and other destinations won't use it.
I live on the Western outskirts of Birmingham, and currently if I want to take the train to London I get a bus and then the tram from Edgbaston Village, alighting at New Street station. With HS2, it would be a further 10 minutes on the tram to Curzon Street.
I would not be inclined to go past a station that has direct trains to the centre of London, in order to go to another station that doesn't have direct trains to the centre of London, particularly if, as I suspect, fares on HS2 are substantially higher.
The other thing about Birmingham Curzon Street is that you can see the site of the station from the existing line from London as it approaches Birmingham New Street. It would be somewhat ironic if you were to stand in a brand new station waiting for your train to the outskirts of London, watching trains go by that are going to, er, the centre of London.
I think would be even worse at Manchester Piccadilly. Why would anyone want to get a train to Old Oak Common when there are direct trains to Euston every 20 minutes?
Some people say that the Elizabeth Line at Old Oak Common would be convenient for onward travel to destinations in Central London. Whilst that may be the case, we already have a thread on these forums saying that Elizabeth Line trains are busy at peak times. I don't think people from stations in between Reading and Paddington, plus those from Heathrow, are going to be pleased if hordes of people from Birmingham and Manchester are going to try and pile on to an already busy train at Old Oak Common.