What would happen if China becomes the largest economy?

Nothing. The US government will continue to waste it's money on "defense" boondoggles and  unwinnable wars.

We're all about to find out. 

He who pays the bills calls the shots!

Just observe the transition from great Britain to United States.

Britain a world power ruled the roost, and lost the lot on the gamble of belligerence with Hitler, only to have to beg America for  "help".

American scooped the British trading network - to become THE superpower.

Bye bye American power!

Bye bye the petrol $ being reserve currency!

Bye bye American embedded institutions waxing American jargon to enrich themselves under guise on "capitalism" whilst not indulging in free and unfettered cut throat competition.

Those countries who want to flourish when they have to choose prosperity of American "humanitarian intervention" resulting in mass chaos or staying in Chinas good books - take a guess who they'll back.

America becomes number two and cannot bully China anymore..

They could get to a certain point to push all the countries to do whatever they want by threatening their economic health. If China can take a bigger hit than say, Americans can.