The wealthy could now have 100s of 1000s in ISAs, tax free, without a single penny being required to be invested in the UK, with the tax payer funding this.
That is then a fault of the legislation that created ISAs, not the people who chose to use them.
I totally agree that the wealthy should pay more to the state. However retrospectively redefining savings vehicles is not going to achieve that without fundamentally changing the nature of our constitution such as it is.
Imagine if lottery winners were suddenly told that "oops, you know that tax-free bit ? We've changed our minds. Pony up" ?