Why pay estimated taxes?
Your logic is not wrong.  But the risk is more significant than you seem to assume.  Essentially you are proposing taking a 2.6% loan to buy stocks. Is that a good strategy?  On average, probably.  But if your stocks crash you might have significant liabilities. In 1929, the Dow Jones dropped 89%.  In 1989, >30%.  In 2008-9, 54%.  This is a huge risk if this is money that you owe in taxes.  If you operate the same system year after year the chance of it going horribly wrong increases.
Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
Why pay estimated taxes?

### Response:
Your logic is not wrong.  But the risk is more significant than you seem to assume.  Essentially you are proposing taking a 2.6% loan to buy stocks. Is that a good strategy?  On average, probably.  But if your stocks crash you might have significant liabilities. In 1929, the Dow Jones dropped 89%.  In 1989, >30%.  In 2008-9, 54%.  This is a huge risk if this is money that you owe in taxes.  If you operate the same system year after year the chance of it going horribly wrong increases.