The break-even rule of thumb
The decision comes down to one question: do your deductions clear the break-even point? The new regime gives you lower rates 'for free', with no paperwork and no proof, so the old regime has to win that advantage back through deductions. As a rough guide, if your eligible deductions and exemptions add up to less than about ₹3.5 to 4 lakh, stay on the new regime. If you have a home loan, max out 80C, pay significant rent (HRA), and put money into NPS, the old regime can still save more. Run both before you decide.
Who should stay on the old regime
The old regime tends to win for people with a home-loan interest deduction (up to ₹2 lakh under section 24), full ₹1.5 lakh 80C usage (EPF, ELSS, PPF, life insurance, principal repayment), ₹50,000 NPS under 80CCD(1B), health insurance under 80D, and meaningful HRA. Stack a few of these and total deductions can cross ₹4 to 5 lakh. At that point the old regime's higher rates are more than offset.
Why the new regime is the default now
The new regime was made the default to make filing simpler for the majority who don't claim large deductions. The standard deduction is now available under it too, and the rebate lifts the tax-free level to ₹12 lakh of taxable income. So a salaried person with modest investments usually pays less tax, with less effort, on the new regime. You can still pick the old regime if it helps you, but you have to choose it on purpose.