I don’t disagree with the Treasury, I am disagreeing with you because what you have posted does not match either what the Treasury or the ONS have published. Even the bit you paraphrased from the Treasury doesn’t relate to the past & present, but is a prediction for the future and furthermore doesn’t say productivity has declined, but that it may “flatline”- flatlining is net zero growth/zero decline.
You stated that productivity has declined, and thus universities are a failure implying this had been going on for years if not decades.
Productivity has not declined, no one has said it has, not ONS, not the Treasury.
Apart from covid lockdowns causing a temporary declune, which is an anomaly and data outlier, there has been no lasting decline in U.K. productivity. There has only been slowing growth in productivity that dates to 2008.
The U.K. isn’t the only country that has experienced a slower growth in productivity. In a lot of ways, this is not unexpected as productivity growth was boosted to a much higher level due the spread of AI and automation into many industries. We have reached saturation for the most part, so productivity gains are diminishing until we hit the next tech breakthrough, and to do that we need a better economy that can invest into R&D.