Will the oceans swallow all of the land?
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.