“China will keep building and anyone can sail by if they want. Eventually they will have enough built for deterrence.”
That is precisely the point, “anyone can[NOT] sail by if they want,” because China has been and still is using armed forces to conquer these islets and to blockade any and all sea and air traffic to those occupied by nations other than China. Right now China is blockading and trying to force Philippines Marines to evacuate one such islet claimed and occupied by the Philippines. This is nothing short of naked aggression by China despite the fact China is a signatory to a number of international agreements and Law of the Sea prohibiting such actions.
Makes the Phillipines the 51st state.
A strategic and resource asset.