I'm not an expert, but here's my take,
I don't really think the US Navy is out there keeping shipping lanes open, just for the US to be a good global citizen? There is some aspect of that for sure, but the Navy is also used as leverage, either implied or actual, in diplomatic relations. Would China stop trade to the US in the south china sea if the US withdrew? No, as its more important to them than the US or the west, but they would have a lot more leverage with the surrounding countries.
Global trade also benefits powerful interests in the US and elsewhere very much, as does spending lots of money on the military. The US Navy is also used to make sure the US dollar is the dominant global trade currency, which has lots of financial benefits as well.
For the defense of actual US territory and citizens, I'm sure 1/3 the current size of the Navy would be good enough, but business/ diplomatic interests span the globe and having the public pay for the security and leverage for these businesses is a good investment.
I know that's not what they put on the recruiting advertisements, and having more than enough to defend the US and allies physical territory, in almost any scenario is still a very useful function of course. But for the US to start expecting others to pay the US explicitly for the Navy's protection of global trade, starts to sound like a neighborhood thug protection racket...
Global goods trade in US dollars is about $48 Billion per day, and there is about $8 trillion of USD per day in currency exchange, and so the estimated value of the benefits for the US, just for that is ~$400 million per day. That doesn't quite pay for all the the Navy but around half.