Imagine for a moment you have a road, It's speed limit is say 70mph.
Now imagine for a moment you build a second road where the speed limit is 125mph and only vehicles that can do at least 60mph are allowed on it, and at the same time you might restrict the old road to 55mph.
Is the second road's only effect to allow you to arrive somewhere a bit faster?
What you've done is you've knocked some time off how long it will take you to get from A to B with the new road for the vehicles that are doing those speeds, but you've still got that older road that can now be used to take far more buses, trucks and other vehicles that can only do the lower speeds (and not having to worry so much about someone coming up behind them 20-50mph faster), or where your speed isn't as important as how much you're carrying in a vehicle.
This is the big thing about building HS2, it's not replacing the old line, it's not just making it a bit faster, it's an entirely new line that is massively increasing the capacity because you can run more slower trains closer together on the old line now, and at the same time run more, much faster trains on the new line.
In reality it probably more than doubles the capacity of the old line, because when you are running a mix of fast and slow trains on the same line you need a lot of time and space between them (and the fast ones can't run at full speed as much), and have to juggle the schedules very carefully, if you're running trains that are all roughly the same speed you can potentially run more trains and it's simpler to manage.
In short it doubles the rail track along that rough route, and the cost of doing it to suit modern high speed trains is a negligible difference between making the new tracks just up to the standard we had in the 80's and are trying to improve on (and by doing it to the best you can now, you're potentially putting back any major improvements/upgrades 20-40+ years, as opposed to having to do them in 10).
A lot of our rail infrastructure is still being upgraded to what was "the new standard" 20+ years ago, so you build anything new to the current best standard (IIRC railway upgrades tend to work in the span of a decade+ because of the disruption and cost of doing it to existing rail).