Will the oceans swallow all of the land? I am wondering why, since the oceans could feasibly hold all of the dry land, they in fact do not.  For example, the Mariana Trench drops to -10.9km, while Mount Everest only reaches to +8.8km.  Further, the oceans cover 71% of the surface.  This causes me to suspect that the negative elevation parts of the Earth could easily hold the positive.  Due to erosion, will the oceans eventually consume the mountains?  What is the timescale involved with such a process, and what forces counteract it?
If no new mountains were built, yes. Ultimately the processes of erosion would render the continents flat, and the seas would be left shallow and filled with sediments. The reason that this doesn't occur is that tectonic processes cause 'orogenesis', the formation of new mountains. The subduction of oceanic crust at plate margins is also responsible for trenches such as the Mariana, and ultimately recycles sediments into the mantle; thereby preventing the ocean floor from filling up.