This might be answered somehow else, but have we passed the "point of no return" of connecting HS1 to HS2? I know the link got canned, but is anything in its way yet, that would make building it vastly more expensive than originally planned?
It felt to me this link could have made Stratford International an actual international station, as trains could have stopped there and bypassed St Pancras without bypassing London altogether.
The answer is yes and no
Yes, currently descoped, no is it could still be done as follows.
One version of the plan had step plate style junctions in the Camden area, it is not too late to include these, as tunnels in that area have not been started. Strictly an access area that will eventually become a ventilation headhouse was started in Camden area, but work on it was included in the pause, so remains in a part built safe stop state.
There is already one step plate style divergence planned (the HS2 tunnels run at different levels near Camden, and the southbound towards Euston splits into two, one going to either side of the platform fan, the leaving Euston northbound tunnel is in the centre of the platform fan). This is to minimise crossing conflicts.
As Euston might now be smaller, it might be possible to still add another junction in same area, even if at this stage no HS1-HS2 link is dug. Many people think it would be foolish not to add junction provision, especially with a reduced size Euston (lack of it will permanently prevent both European services getting to Birmingham, and some services from Midlands and North being extended to Ebbsfleet or Ashford for International connections).
HS1 has provision for a spur towards the north London line at Camden Road, and from memory one of two tracks was laid. It might be possible to instead build a diving ramp towards HS2 from these spurs. Wouldn't be high speed (as in 100+mph) due to curvature, but as trains on HS1 run relatively slowly as they curve into St Pancras, and trains on HS2 will only be about mile or two from Euston they won't be at very high speed either, so even a link line at about 50-60mph is not going to eat capacity (at least not whilst less than around 18 trains per hour is usage rate of HS2).
Really it is a question of money, in two stages, build junction provision, then second phase build link. Technically from construction point of view, there is no reason cannot be done.