You should arrange to visit a train planning department, then you'll realise that your suggestions are meaningless.
Even Digital Railway's information talk about uptown 40% capacity increases.
Although like most things that say "up to" I would take that figure with a pinch of salt.
However, for now, let's accept that it's achievable. Rail growth between the West Midlands and London and the North West and London has gone up by between 65% & 70% between 2009 and 2017. Which puts it higher than was expected by HS2 for the opening of Phase 1 (52% growth from 2009).
However what isn't well known is that between Birmingham and London growth was expected to be just 1% per year, because this market was seen a mature. However if this were the case then the 67.5% growth seen between the West Midlands and London would be even more spectacular if Birmingham wasn't contributing much of it.
As to that the 71.5% growth seen between the West Midlands and the North West (2009-2017) and you can see why something needs to be done and soon.
Even if we implemented digital railway principles then chances are we'd still need to do something else soon anyway.
By getting on with HS2 we can use digital railway improvements to cater for the extra growth beyond that expected in the model.
It then allows us to use this extra capacity released for extra more local services once HS2 has been built.
If we don't build HS2 then we'd need to do something that looked very much like it (or elements of it) fairly soon anyway.
What's the alternative?