America would only get weak if it fell behind technologically. Demographics are a challenge if people are not adequately assimilated enough into society and allowed to be productive members of society. In terms of demographics, the US will probably have a younger average population and attract top talent more than China for the foreseeable future. This is precisely why, should the US actually reoriented to compete with China, it won’t build the best infrastructure but it will be a robust effort in catching up where it has fallen behind and surpassing China where needed. All this is dependent on domestic politics as well, and that is another place that could slow America down; the political infighting.
The US economic culture is considerably different from that of Mexico and Brazil; it’s a Western European business tradition versus the Spanish/Portuguese system which has been holding back Latin America for hundreds of years.