The turmoil in Eurasian continent and Population explosion in Africa when you compared to improved peace in Americas. I will bet by shirt on Americas in general and USA in particular. No, Sir China is not going to touch US in military terms during my life time and probably have some economic benchmarks they may dominate but per-capita will suck. For all the hew and cry, China is middle-income nation with 17 neighbors, Middle East even with Petro-dollars in basket case, Europe is demographically falling and struggling with Immigration.
America will remain the power to reckon for next 50 years if not more.
Most people believe that the US will always have the better fighter planes, the better missiles, or the better tanks. In reality, we are about to be parity. They will be stronger in some aspects, we will be stronger in others.
If you don't believe me, go check out the Chinese J-20, J-31 aircraft. Their DF-41 ICBM. The Type-99 tank.
They have been catching, and fast. They already have some systems that are better than ours. They also have twice as many soldiers/sailors/airmen/marines.
Source: Former Intelligence Analyst.
But if I you were interested in countries and power, I would not focus on J-20 or J-31 but the current purges and anti-corruption drive in China. The most interesting question about China in 2035 is, how will communist party hold power now on.
I don't know where you're getting the notion that the Chinese navy doesn't compare to Japan, because its stronger/larger in every measurable way. Are you accounting for the 30+ ships the Chinese navy has been building every year recently? I would agree with that statement six years ago.
You are absolutely correct: The most interesting question is can they sustain their current output. Will the party hold up?
Economic battles are fought through puppets (Syria, Iran/Iraq, Koreas, etc.), not between the major powers themselves, thanks to nuclear proliferation.
China has more basic problems, their coastal provinces are prosperous and their interior is not, and they have their own centrifugal forces. Long story short, you are overestimating america's problems and underestimating China's problems.
Before WW1 and WW2, US was indeed a 2nd tier nation but they were quite happy.
And don't forget, corporate masters are everywhere, not just in US.
The only people that truly believe the US is in decline are Trump voters, neoliberals, and survivalists.