I get what you are saying , there are a few ways of validating and invalidating
tiles/triangles as walkable , there are the walkable/non walkable flag of triangles
which could be changed but yes it must rebuild the navmesh ,and there is
the tile remove and tile build functions which also needs the navmesh rebuild
but the whole navmesh does not need to be rebuild the small area blocked
by an obstacle could be rebuild by only using its bounding box with a bit of exstra padding
which means you could rebuild that area when you add or remove the obstacle
but only that small area not the whole navmesh ....

All in all rebuild is needed but you dont rebuild the whole navmesh only
the area you want to ... But i will look futher into this perhaps there is a way
to alter something that would filter out tiles/area from the pathfinding
in a faster manner but im not sure there is a way because mikko would
surely know .. I thought about looking to find cost values