I find it interesting how few people recognize the asian situation from the asian perspective. Japan, Korea, the PI, Thailand, Vietnam ALL have a LOT more options than US or no US. They can just as easily comply and submit to the Chinese by way of not interfering with Chinese expansion. For an extreme example: Imagine what would happen if Japan decided that the US can't use offensive forces from Okinawa anymore. Yes, that's an extreme example that IMO will NOT happen, but options like that do exist for all these countries.
Consider what would happen if the PI came to it's own agreement with the Chinese where they simply refuse US basing rights in particular places for mild territory concessions by China. All these countries have their own agendas, and they all view 'sovereignty' in a very different way than we do. The list of these kinds of 'what ifs' is endless.
In general, if they observe the US taking steps away from treaties, agreements or promises made in other places, their confidence in our will to come to their aid could degrade, driving them to seek other options to secure their own interests. It's very dynamic and every single one of these countries has their own interest in mind, and being allied with the US is far from being a catch-all solution from their perspective.