I think your suspecting what if case scenarios, where by mistake someone removes a subnet from one region. Well its possible, but your design should cover those scenarios too.
Often the design is built in such way, assume if EAST pool is down, Users of EAST region are sent to WEST pool automatically, this is done with weightage/score.
Assume if user is coming from x region, but not part of EAST or WEST regions, because for xyz reason its not matching, design has to be built in such way that even if users are coming remaining other regions, they have to be sent to default/fallback pool, you can name whatever you want. Here you can either you NOT condition or use the 0.0.0.0 option.
Try building the infra in your lab and test it. Let us know if you need some inputs.