A changing of the guard among the biggest buyers of US Treasuries has Wall Street veterans bracing for further pain in the world’s largest bond market. Increasingly absent are steady-handed investors including foreign governments, US commercial banks and the Federal Reserve. In their place, hedge funds, mutual funds, insurers and pensions are piling in. Market watchers are quick to note that unlike their more price-agnostic predecessors, the new buyer base is likely to demand a heavy premium to finance Washington’s spendthrift ways, especially with debt sales set to surge as deficits swell.