50,000 un-invested in your savings bank account, which earns 4% interest annually, for 15 years. You will have Rs. 90,047 at the end of 15 years.
As you can see, in Case 1 and Case 3, the amount earned in much lesser than what is needed to maintain your purchasing power, ie: Rs. 2.08 lakhs. Even an investment yielding 8% post-tax return does not look attractive, simply because of the exceptionally high inflation rate of 10% per annum, which has eaten into the value of your money. So you must earn a return of at least 10% per annum, post-tax, to justify the increase in expenses over the 15 years. Note that in our example, only an equity investment in Case 2 gives you a corpus which is sufficient to beat inflation.
This is not to say that you must invest only in equity instruments. Equity, as you know is high-risk and you must not invest your entire corpus in this. You can choose to have a mix of high quality debt instruments as well as equity, to match your risk-return expectations. Remember to invest wisely by keeping in mind the inflation factor, which is the most important determinant in financial planning.