The Trillionaire Savings Calculator is just for fun, as you'd need a pretty high interest rate to achieve this kind of return! However we can all dream, so have a play and work out how many years it would take you to become a trillionaire with different saving amounts and interest rates. For example, if you were able to save 10% of a £30,000 salary, at a total interest rate of 2.45% (after tax on your interest has been accounted for), it would take you over 657 years to become a trillionaire.
However, chin up. You can still make some decent returns from saving up to your ISA limit each year, because you will not have to pay any tax on the interest that you earn. The beauty of an ISA is that once your money is in that 'wrapper', you can keep it there indefinitely, year after year, and can move it from one provider to another to ensure that you are still getting the most competitive interest rate on the market. Just remember that you cannot take any money out of each year's wrapper without losing the tax benefits for future years, so aim to lock the money away for as long as possible.