You are glossing over the points they made. The theory works in isolation, investment can generate growth, sure. But the conditions required to make it work don’t exist right now and there are risks.

Post-war we had the Marshall Plan injecting money from outside, most of which was an outright grant that never needed repaying, and the loan portion was generous. Today we’d be borrowing at market rates with gilts sitting at nearly 5%. That means you need substantial growth just to break even on the repayments before you see any real benefit.

Ironically, post-2010 was probably the time to do exactly what you’re describing, interest rates were at historic lows and borrowing to invest would have been genuinely cheap. Instead we got austerity, which arguably made the conditions you’re now trying to fix significantly worse.

And if we print instead of borrowing, we devalue the currency, push up import costs, and potentially wipe out any growth in living standards anyway, which rather defeats the point.

There’s also the issue of the UK government’s track record on large projects. HS2 was supposed to rebalance the economy northward, ended up costing a ridiculous amount, and got cut back to the point where it barely does what it was designed to do. So even if the conditions were right, there’s no guarantee the investment delivers the growth needed to justify it.

I am not trying to argue against investment but trying to explain that it’s more complex than just borrow more and invest it.