I think it would have been a better solution for HS2 to terminate at King X St Pancras, even if through HS1-2 services were not provided. But that bird has well and truly flown. When the Kings Cross area was redeveloped leaving room for rail expansion would have been sensible. Yes I know its only a 10-15 minute walk from Euston to Kings Cross, but if you have luggage/dont know where you are going its probably less attractive.
It got a good airing in a thread here a while back.
Consider the connectivity from Kings Cross compared with Euston, Kings Cross St Pancras has Domestic HS1 services to Kent, Eurostar, 3 deep level tube lines and the Circle/Met/H&C, Thameslink services to Surrey, Sussex & South London. ECML services, local and long distance, Midland Main Line again local and long distance.
Had there been the vision and political will Kings Cross St Pancras could have become a "London Terminus" with the addition of HS2 meaning that it would become the terminus for everything north of London. Crossrail would probably have been routed via Kings Cross as well giving connectivity to Heathrow, with Gatwick already served by Thameslink. But all this is firmly in the category of "what could have been"
As someone who has travelled through Kings Cross for close to 60 years the improvements over time are most welcome but that doesn't detract from the feeling that it could have been better, anyone who used this area in the late 70s and early 80s in particular will remember the issues.