As well as all the (huge) benefits of HS2 - which as mentioned were never reducible "15 minutes quicker to London from Birmingham" - it's yet another cut that will make it more expensive in the long run than if they'd just kept going. We keep spinning up huge teams of people to work on stuff like this and then never having anything for them to work on afterwards. Part of the Crossrail money went to founding an engineering and construction academy for workers, and the idea was they'd go straight from 1 to 2. But of course 2 is now also punted into the long grass, and most of those people have scattered into other work and it'll be much harder to get a new team together with the requisite training and experience again if/when 2 does happen. Same with the architects/planners/managers. There aren't that many people with experience in overseeing these kinds of megaprojects, but if you're one of them why would you want to stay in the UK when there are other countries you could move to to work on their equivalents?
Look at France. Paris has already managed to build 5 Crossrail equivalents and has two more metro lines under construction, while even small regional cities have their own metro lines - Rennes just opened its second line, a city around the same size as Leicester. If you keep building rail infrastructure it becomes cheaper-per-km with time because you also build up the institutional and industrial foundation for it. Every time it looks like something similar might happen in the UK, it gets kneecapped for "ballooning costs". For decades the Treasury has prioritised five-year return on investment, making London the only really viable location for new rail on paper, and funnily enough the end result of that has been a massively lopsided country economically. One of the world's most productive cities, now carrying cities and towns less productive than those in many ex-Soviet republics around on its back.
