Will the ocean eventually erode all land?
Continents are made up of different, lighter materials than oceans and both continents and oceans "float" on the mantle. Continents are also thicker than oceans so erosion does not only need to take care of the ~5 km which is above sea level but it should also erode away the continental root (which can be several 10's of kilometers until the continental crust is actually slightly thinner than oceanic crusts are today.

However tectonics is not the only source of topography which can easily be seen if we look at Mars where a hotspot has created an enormous mountain on a tectonically inactive planet.

So if you want to end up with a topography free planet you need to shut down all possible mantle plumes too.