I think a plausible scenario for the Philippines if China gains an upper hand in a, what I hope never to happen, armed conflict between USA and China over Taiwan is that the US forces in the Philippines will instigate a putsch to the government at the time, regardless of who it is, establish an interim government (ala Vichy) in the name of protecting the whole of Philippines against any (likely imagined) separatist elements seeking to destabilize the Republic of the Philippines. This way if China wins the conflict (again, hoping that conflict never happens), then there is always a beachhead for the USA to harass and otherwise somehow keeping China's attention. So this is an insurance policy.
However, another more sinister scenario is that if the US loses bad enough and can no longer pay attention to that side of the world, then the remaining US forces can potentially turn rogue, become a warlord that is much bigger and powerful than the Philippines military itself, take over the norther part of the Philippines and finally declare itself the rightful governing body. It will then signal non-aggression and non-contention in SCS with China, so China doesn't mind having a neutral and dependent state in SCS. And since US at that time is too preoccupied with losing and its aftermath, it will be a while for it to contest facts on the ground in the Philippines, And in that interim the new regime in Manilla can come up with political palatable ways to pacify USA, and with the US not wanting to engage yet another armed conflict in SCS in full view of China (who had won and took over Taiwan), it will probably accede to the existence of the new government now ruling from Manila since it is of US origin and mostly Americans anyways. It can always sell the new regime in Manila as an offshoot of the USA and a win out of the conflict.
The bottom line is, the US force in the Philippines is just as much a force to counter China, an insurance policy for the US if things go side-ways and worrisomely, a huge potential warlord in the Philippines. The latter two possibilities are existential threats to Philippines itself.