That is partly true but it's a downstream consequence of the ultimate cause. Those running and pushing the project in the early stages desperately wanted it to be a 400+kph route for "prestige" reasons.
If they hadn't been so focused on that, they could have built the new rails largely alongside the existing West Coast Main Line which would have been quicker, cheaper and much less controversial.
But because they wanted those very high speeds, the route had to be flatter and straighter than the existing WCML, which means many more tunnels, longer and more complex construction, more people objecting, inconvenienced etc and ultimately much more cost however you look at it.
If they hadn't been so focused on that, they could have built the new rails largely alongside the existing West Coast Main Line which would have been quicker, cheaper and much less controversial.
But because they wanted those very high speeds, the route had to be flatter and straighter than the existing WCML, which means many more tunnels, longer and more complex construction, more people objecting, inconvenienced etc and ultimately much more cost however you look at it.