The biggest benefit is capacity, with the fast intercity trains running on a new line you have much more space to run more local and freight services on existing lines. Due to the current difference in speeds on the same lines, the capacity is limited even more because trains have to be spaced out more to stop them catching up with each other, so you get more than one extra trains capacity on the existing lines for each intercity train moved off.
With regards to WFH, it’s not as big of an impact as you might think, and even those working from home still travel. Rail usage is mostly back to how it was pre pandemic, but with some changes. There’s now more leisure traffic than there was before, and fewer commuters.
It’s really not a good idea to stop at this point. We’ve committed to the most expensive part which will be under-used without the rest. The tories have had more than a decade being in charge and they’ve done nothing to try to control the costs, while also trying to reduce risk which then increases costs. There’s also the myth that has become pervasive recently that government debt works like household debt, which isn’t true. Most government debt is owned by the Bank of England, which is controlled by the government. In harder economic times the government should be spending more to stimulate the economy, not continually imposing austerity.