That is a very good question actually. Its not clear at all.
First counterpoint is that is a strategic position they’ve got, such that you really dont want someone else (China, say) operating from it. You and tje other wealthy characters in East Asia need them, if you are going to defy the Chinese.
Second is that the economic benefits from the US alliance are at this point marginal. They are marginal now. Foreign investment there isnt mostly from the US.
Third is risk -If the US is a declining power or will be scaling back its international profile, the US link will be perceived as more a danger than as insurance.
Is Duterte making his peace with Beijing?
If Trump is elected, the international perception of America as feckless will be smashed, just as in 1981 when Reagan became President.
Duterte might want to dial back his defiance a couple of months.