I disagree on every count.
1) current Avanti fares are only eye watering if you choose to travel in peak periods on a flexible ticket. You can buy a ticket for Birmingham to London, for travel tomorrow, for £38 return. That is simply not eye watering for a 225 mile round trip on a journey that takes 1hr30 each way. (Less in normal times).
2) HS2 will have a lot of seats to sell, as
@Bletchleyite correctly says. You can expect lots of cheap tickets
3) in my opinion, when HS2 is open, the cheapest options for LNW / Chiltern will go. In fact I suspect they will go for Birmingham to London journeys well before that. They are not competing with Avanti any more.