Will the United States (or its
successor institution) still be the dominant force in the 22nd
Century?
I think the answer depends upon which attributes of the U.S. you believe brought us to our current state of global dominance. Let's start by agreeing that commercial dominance is at the core of American power. World powers have always been based on their commercial base. That moves the question to an examination of attributes that have generated the dynamo of commerce we have experienced for the past sixty years.
Commerce thrives on six attributes:
diversity, meritocracy, openness, markets, technological affinity and
decentralization. I argue that these attributes are deeply built into
the structure of America and will, thus, support the commercial
dominance of the U.S. for a long time. (My book Commerce explains why
these attributes favor commerce.)
Diversity is an inherent American
quality. Founded by immigrants from every part of the globe, the
inflow has rarely slowed. The anti-immigrant tide has always
been held at bay by the needs of domestic business. Improved
technology may make illegal immigration more difficult, but, like money
and cocaine, the demand will always bring a supply of immigrants
into the U.S. No other country is the magnet for immigrants like the
U.S. No other country is as able to accommodate immigrants so
readily. (Canada and Israel are both a close second.)
Meritocracy grew from the start in the American revolutionary war where ordinary men were valued for their merit and not their status. This attribute was fed by the massive immigration into the American west where men and women could escape status-burdened towns and cities to start anew. The Civil War saw the more meritocratic North defeat the aristocracy of the South. The struggle for meritocracy to remain dominant in American will continue. The near total absence of an existing birthright elite is good insurance. Again, only Canada, Israel, Australia and Japan are meritocratic societies.
Openness is a synonym for honesty. The
U.S. ranks below Japan, Holland, Denmark, Israel and Sweden on this
attribute. But that is still high enough to keep open markets
functioning. The federal government has been improving the level of
American corporate openness and can be expected to continue doing so.
Markets are generally open and
unrestricted in the U.S. Monopoly conditions have been steadily
diminishing for decades as global trade accelerates. The U.S. was a
major force in creating the WTO and will continue to support this
global direction. The physical size of the U.S. will make monopoly
conditions hard to re-establish.
The U.S. has no particular edge in technological affinity. Only Arabs, Latins, Africans and other tribal peoples seem to lack the technological affinities of the rest of the world.
Decentralization is the birthright of
America ... an attribute found nowhere else. 
The American form of
government is always self-organization from the bottom up. First a
water district, then a sewer and electric district, then a fire and
police district and finally a school district. Each is formed by a
commission of local people. Americans self-organize from the bottom
up. The commercial value of decentralization in the form of
initiative and autonomy are at the core of America. It is also what
protects the rebellious, defiant sources of creativity in Americans.
The American global commercial megalith would seem to have a long future ahead of it.