HS2 is an investment that should last until railways as a major form of travel are redundant. Spending maybe something like £300m+ on a new mini fleet that can't arrive before 2022 and will not be needed after 2026 is just wasting money.
If the Camp Hill Chords are ever built then there would need to be significant investment in Moor Street. Curzon Street won't have conventional rail services and will be litterally next to Moor Street anyway. If the chords are not built then it will be a matter of squeezing as much capacity out of New Street.
I think that you're missing the point, if you built 6 coach units then they would probably be of more use after phase 1 of HS2 opening than the 9 or 11 coach units, as they would be able to serve two places with one path. As an example there wouldn't be the demand for an 11 coach train to Manchester or Liverpool asking the WCML, so if you could run a 6 coach train from London to Crewe and then split it then you could run another service on the southern WCML which you can't currently do because you need two paths to serve those two cities.
In fact, I wouldn't be surprised if it resulted in some of the existing 9 coach units being shorterned to be 6 coaches to maximise these kinds of services, which would mean scraping 3 coaches but would be a lot less wasteful than scraping whole units.
I've also pointed out that in 2026 there could be a use for some of the 390's on XC on Manchester to Southampton services (with Bournemouth being served by the Newcastle services).
As such the new tilting trains would still have a use after 2026 and even the longer 390's could still have a use and if not then by shortening them (and scraping some 25 year old coaches) they could still be useful. Even if you converted ask the 9 coach units to six that would only be 66 coaches (or about 15% of the number of coaches which Virgin first ordered) that would be gotten rid of, on trains which will be over half way through their life.
That's all assuming that an open access operator (I'm looking at you Alliance) doesn't then take on some of the 390's for their services. For instance to replace whatever 110 mph stock (which could be relatively old Mark 4 coaches and locos) they use on the Alliance's Blackpool services. As they would certainly benefit from 125mph rolling stock as well as stick which may last a bit longer (as even if the coaches could be good for longer the Locos may need sorting sooner and so the 390's could be a good option.
Whatever the future, I'm sure that there's a future for the new units as well as for (at least most of) the existing 390's. As even if not sure are added to the existing services there's not going to be so many added that being able to run at 125mph wouldn't be useful.
Also as one final point, if you look at the rolling stock that SWR are getting to replace their 75mph units, it's 100mph. Why? For the simple reason that even if it rarely runs at 100mph there's some services which do and so it's easier to be able to use any unit rather than having a micro fleet that is locked to certain services. Likewise at WCP what would be better if a 390 fails and the only spare units are new units? Would you rather:
A) the train runs but with slower units so that you are late and it may have an impact on other services as well
B) the service is cancelled
C) the service can run, just formed of a pair of units but with no time penalty
Most customers would be happiest with C. Likewise most TOC's especially if there's already a small delay due to having to get units from a depot to run the service, something that tilting trains could recover from a bit with their higher top running speeds whilst slower trains wouldn't and may loose more time (having a bigger impact on the services following).