Why can't I keep buying US index funds after I move back to India permanently?

Two problems arise. First, most US brokerages restrict or close accounts held by non-US residents; many won't accept new purchases once your address is in India, and some require you to liquidate or transfer. Second, from India's side, a US-domiciled mutual fund or ETF you keep buying becomes a foreign asset with Indian reporting and potentially unfavorable tax treatment on distributions and gains. You can often still hold existing positions, but adding to them is where accounts get frozen. Before you move, check your broker's policy for Indian-resident clients, consider consolidating into a US brokerage that services expats, and plan whether to continue US-market exposure through India-available routes instead. Sort out custody arrangements before you land, not after your account is restricted.