They could remove the ISA shelter at any point as well though. For the vast majority of savers (although possibly not readers of this board), that would probably make no difference to the tax they pay.
According to https://www.finder.com/uk/saving-statistics, the average UK adult has £6757 in savings, and the average in Adult ISAs is £6049. It's not explicit on that site, but I assume that the ISA figure is included in the total savings.
At current rates, for basic rate taxpayers you need to have around £100K in taxable savings to use up the personal savings allowance. I haven't found a graph showing the propoertions of people with savings over that level but I suspect the numbers are relatively small in compariosn to all the people below that level. It might not be that politically unpopular to scrap ISAs and increase the personal savings allowance by say another £1000. Most people wouldn't lose out, and the Treasury would presumably gain a bit of tax from the minority, and the bureaucracy of ISAs could be scrapped.
I was going off-topic there, so I've started a new thread here instead. What do you think - could/should ISAs be scrapped and the PSA increased? How likely is that to happen?