It doesn't affect you if invest a proportion in shares,
I think that reeves has been seduced by the financial markets to try to force people to buy shares. The impression given is that this is investment in businesses which will promote growth. But it isn't.
There are basically two purposes for share trading. One is the sale of shares to either capitalize a new company, or to raise additional capital for an existing company. A primary market, so to speak.
The other is the secondary market which handles trade in existing shares.
If shares are bought in the ,primary' market the purchase money goes to the company. So, direct investing in a (potentially) productive company is likely to contribute to growth.
In the secondary market the purchase money goes to the previous holder of the shares. The company which originally issued them doesn't see a penny of it. Basically, it is a market for speculators. Shares are either purchased with an eye to selling them at a higher price, or in anticipation of high dividends. This is not productive of growth. It may make some people rich, it pays high salaries to traders in the financial markets, but it does little to help national growth because, as I say with wearying frequency, the rich have a marginal propensity to spend. In other words, very little of this money even reaches the domestic market.
It's also quite risky because, as we should know, share prices can fall as well as rise. So can company dividends.
When I consulted AI on the proportion of 'capitalisation' shares sold/purchased to the proportion of speculative purchases traded it reckoned a rough estimate of about 5% of share trades are for capitalisation, 95% are secondary trades (speculation)
So swapping investment in a cash ISA for 'investment' in shares is much more risky.
Now, if she had offered concessions for purchase of shares in the 'primary' market, it might have been helpful to encourage new business start ups and lead to potentially more growth. But that's not the effect of most investment.
And, of course, a disincentive to new business investment is the fact that a very significant number of consumers are short of spare cash. No use starting a business if no one is going to buy your product..