It's not just for China's benefit, it's for the benefit of the liberal (free) trade regime, which in turn benefits everyone (including the US). That said, this situation cannot last forever, as at least theoretically, the market will demand higher interest rates as an incentive to push more capital inflows into the US (another way of saying financing the trade deficit).
Why has it lasted this long? Because the US has the reserve currency, and is able to monetize its debts.
Why does the US allow this? Because as long as trade is free, the US can grow its exports, and thus generate revenue for the economy, and shrink its fiscal deficits. But with the left wing controlling the levers of power for decades in the US, our fiscal situation has greatly deteriorated. Economic reform is not a realistic proposition at this time, so the fiscal deficits are not shrinking as fast as they need to. But when both the trade deficit and fiscal deficits can no longer be controlled, the result is trade barriers to reduce the trade deficit, and austerity to reduce the fiscal deficit. Result? You guessed it, the Great Depression II.
For a great case study on the British Empire going into the Great Depression, please see:
Retreating from Globalisation: the British empire/commonwealth experience between the wars
This is why there has been talk of "rebalancing" the Chinese economy for a few years, now. As you stated, the investment-based economy no longer works. Moreover, it can't last much longer, because those investments are essentially being financed by America's trade deficit. In the 1920s, Britain and America couldn't solve this without austerity and trade barriers, which caused the Great Depression. When Japan was dominating in the 1980s, the Plaza Accord was executed to try and solve this. It saved the US, but caused a massive real estate bubble in Japan, which it is still recovering from to this day.
Will China be able to smoothly assume leadership of the liberal trade order from the US, or will its mercantilist policies essentially cause the liberal trade order to grind to a halt again? China (and the rest of the world) can't afford for the US to collapse, so we all need Xi Jinping's reforms to be successful.
I hope you can see that I fervently wish China to succeed.