If the terribly ugly (in my opinion) nation nal library was knocked down and relocated, why not have the old Sommerstown goods yard as the main HS2 station adjacent to st pancras and kings cross, with a through connection to HS1??
1: the site isn't long enough, so you'd need to demolish the Francis Crick Institute as well, along with some quite expensive private properties along Brill Place and towards the old church.
2: the aforementioned owners of the aforementioned buildings probably wouldn't be too happy that they were being turfed out at such a late stage
3: the previous property owners around Euston would probably be quite miffed that their demolished properties now needn't have been demolished
4: you'd need to find somewhere else in central London to build a new British Library, taking many years to build, before you could start moving the archives over and then demolish the old one
5: the plans that have been around for ages, and are currently under construction, see the new station at Euston, and changing them would be a bit of a bother
6: all of the above combined would require spending a lot more money, over probably a decade, just to get to where we are now