No one will replace USA neither China neither any other country.
Because world will not remain Unipolar.
Non of the other country will be able to match the same level of economic threshold around the globe which US enjoyed for past 30-40 Years without any real competition.
Multiple big economies are going to appear in future which influence thing's too.
But in security dimensions it can be China which can try to implement it's own laws and rules but to counter that Quad was created and US also understand this that's why they became the member of the Quad but their position in Quad is not that clear.
There is one little problem with China;
they can't differentiate between friends and foes.
When Rodrigo Dueterte came to power in the Philippines, his first move was to junk all the ties with the USA and build ties with China. He massively liberalised the economy of Philippines and rolled a red carpet for Chinese investments. At one point, it was so bad that the local businessmen there started getting angry about the special concessions that Chinese corporations were granted.
Even during the COVID-19 pandemic, it was seen that Chinese factories in Philippines were allowed to keep their outlets open while Filipino businesses were ordered to remain shut. This level of boot-kissing was only next to Pakistan's. As you can see from the recent reports, CCP continues to take over Philippines' excessive economic zone.
Similarly, when the Human Rights violations issue was raised in the UN during the initial days of the Uyighur issue, dozens of African, Asian and Latin American countries supported China - many so-called Muslim ones as well. All this was because of the Chinese investments. But then what happened? Did China forgive their loans or give them extensions? Nope. Indonesia, Malaysia, Brunei all have tried to cosy up to the CCP and yet, they continue to get hammered.
China's sense of exceptionalism is a lot different from the United States - the latter has this sense of exceptionalism that gives some form of leeway to its lackeys like Japan, South Korea, Germany, the UK, etc. The US is kind enough to throw some scraps at its pets.
However, China's sense of exceptionalism is more reminiscent of Imperial Japan where they think it is their god-given right to rule everyone and if they submit to them, it is just something they are expected. And that is dangerous. With the USA, there is always somewhere you can have some form of negotiation if you are their lackey (not implying that we should be); but with China, nothing short of tissue-paper treatment is acceptable to the CCP.
The only option for a rogue tyrannical state like that is to be put down.
As for American decline - it has long started. The US will never be lowered or challenged from the outside. With the advent of the Biden-Harris Administration, the decline will only get worse. Donald Trump was trying to save them from the typical Deep-State nonsense that the US was living in until now. That's why his presidency was so troublesome as the Deep State couldn't continue their agenda under his non-conformist, anti-establishment and of course eccentric style of administration.
The US is definitely not a democracy and there is more to their government structure than they show the government. Call me a conspiracy theorist, but something is off about their system.