^Beaching a ship to serve as an outpost is an extraordinary measure. Doing so would invoke a reaction. And the first two times it was done was in response to the Mischief Reef incident.
While old hulls can be used for this purpose they would have to be re-commissioned as the fact that the BRP Sierra Madre is still on the roster is one of its biggest protections (any actions against it would be essentially and act of war, subsequently invoking the Mutual Defense Treaty).
If the Navy were to intend to use some of its old hulls for that purpose they would have to be kept in dry dock or storage to keep them structurally sound and seaworthy enough to be be moved. But that is a matter of policy for the Navy. Consider that the reason for the decommissioning of old vessels is to free up manpower and resources to allow for the acquisition and introduction of the newer, more capable assets.
Lastly, in the past the weapons from the decommissioned ships have been turned over to the Philippine Marine Corps,
which has refurbished them into AAA platforms.
