ISA savings do not threaten the viability of banks and building societies.
Nor do people saving with the government by buying its bonds threaten the government’s financial viability.
But we call people saving with the government ‘borrowing’,
What government is doing there is borrowing. Just as it is indeed true that eh savers are saving. We also say that the banks are borrowing from the savers. That’s why a savings account is a liability to a bank, why a loan outstanding to a borrower is an asset to a bank.
Because there are two sides to any such transaction and whether it’s an asset or a liability depends upon which side of the line we’re describing.
And people do tend to get a touch tetchy if they can’t regain their savings, even though they are just the bank/the government borrowing. As those lines outside Northern Rock showed.