The problem with non-termini solutions for HS2 in London is that to gain the most benefit from them, you need to have two equivalent networks to connect together. Thameslink works because the MML/ECML slows are not a bad match at all for the line to Brighton, as is the pairing between the GWML and the GEML in Crossrail. On the other hand, HS2 simply does not have an adequate paired line because it alone will carry the majority of true LDHS services in this country. HS1 is a very different beast and is actually not a bad pair for the WCML fast lines once HS2 is complete given that only domestic services could be paired. Also, a series of different options were explored back at the start of the HS2 programme and the idea of completely rebuilding Euston was the least bad solution of all. Much of this is because a complete rebuild is necessary in any case and actually, being able to expand the station outwards to have more platforms during construction would not be a bad idea whatsoever.
On the other threads, one sees the doubt about the real suitability of the suggested London terminating arrangements. I will give you (and others) this, namely that it seems high time we stopped thinking in terms of termini, perhaps. That's the whole basis for Thameslink, after all.
