What did Alexander Hamilton fear would happen if the proposed Constitution was not adopted?

Basically, Hamilton feared that the country would be
broken up into a lot of semi-independent states that would be run in such a way that the
country's economy would suffer badly.

Hamilton wanted a big
country with states that were not very independent.  He wanted trade to flow freely
between the states.  He wanted it to be hard for the common people to take control of
the government, preferring that elites have more
control.

Hamilton feared that without the Constitution all
the states would be practically independent and would not trade with one another
easily.  He also worried that the common people would take over the state governments
and enact bad (in him mind) laws like the ones being demanded by the rebels in Shays's
Rebellion.  Between these two factors, the economy of the country would be ruined.  The
US would have ended up as a loose confederation in which both the states and the country
as a whole had very weak economies.