I think it's safe to say that any hope for a Regional Eurostar and/or HS1/HS2 connection is dead in the water (especially since HS2 originally had most of it's paths dedicated to domestic trains). For what it's worth I think Crossrail 2 would be a better investment since Euston St. Pancras could provide a long corridor or travellator for passengers arriving from the north and wanting to transfer without having to lug their luggage down Euston Road.
I think the longer HS1-HS2 link is definitely dead, but a short spur from Camden viaduct at end of HS1 to a subterranean junction with HS2 is still possible even if it is restricted to 50-100km/h. The low speeds aren't really a massive problem as trains will be close to London terminus, so all trains are unlikely to be doing more than about 120-180km/h.
Whilst Eurostar might not be interested, I still don't think anyone can rule out some of the potential low cost operators operating a token number of trains to Birmingham or Manchester.
My reasoning is that some are happy to block book parts of the train and sell to a holiday tour company, and of course some of the low cost airlines serving UK do the same, allowing holiday groups to book multiple seats on their flights. I know some people would say no one will want a 7 hour holiday journey, but how many people get up at 3am or stay in an airport hotel, because check in is about 4-5am for low cost flight, or the return gets back late evening, too late for public transport home. Is a long journey worse than these options if it avoids airport hotel ?
Also by Late 2030s or 2040s when line might be available, the low cost operators will also have access to many multi voltage trains that are being demoted from front line service. So could potentially be fares nearer €99 (say £85), not the expensive no competition prices there are now.
I totally agree won't be a business market for trains to continue north of London, but I disagree that a few through trains carrying holidaymakers from UK, or tour groups from Europe won't exist as token number of through trains, for convenience of travellers