Changing ISA's is another of Reeves stupid ideas. On the contrary, if she wants to encourage saving she needs to have tax efficient offers on hand for small savers.
The most basic financial advice says that if you only hav a small quantity of savings they shoukd be saved as cash.
If she wants to encourage stock market investment and getting more investing in it safely, what she should do, is put a cap on how much you could put in cash ISAs in total - say £100,000 - which might be saved up over 50 years by saving £2000 a year in a cash ISA, or over 5 years at £20,000 a year.
If Reeves just reduces cash ISAs to, say £5,000 a year, many, if not most people with cash ISAs will not then buy Unit trusts or similar, they will simply put the remaining cash in the Building Society and manage without the tax free wrapper.