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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.