So you're saying doing the same thing repetitively wouldn't make a difference ?
Explain the pickaxe, or cutting a tree, or erosion.
pickaxe will shave the surface but it's a limit to how much you can shave with it. Once you've broken all the weak surfaces you are left with only solid rock. From there on it's not the force from the pickaxe that does the work but the absence of forces compressing the rock. This can never, ever happen with a body with a gravitational field like earth's.
Cutting a tree is a complete different physical principle and to be honest can't even be compared to what we're talking about.
Erosion shaves the surface, but the majority of the work in erosion is made by chemical exposure, i.e. oxidation, decay and putrefaction of the matrix of the minerals. If this would not happen then the rock would not suffer erosion. The common belief that water and wind force slowly shave the rock is a simplification used to explain erosion to people who do not understand geomechanics. Thankfully it so happens that I'm a Geological Engineer and my expertise PhD is on the field of Rock Failure and Applied Geomechanics.
You're a dumbass. You never see NGC? A asteroid as big as everest is enough to kill all life on earth. So a asteroid as big as japan would smash earth.
Note that annihilating all life on earth is relatively easy. All it needs is enough energy to turn the surface of the planet into a smoldering, melting matter scene. For example the asteroid that killed the dinosaurs had a surface of about 250 sp miles, roughly the area of Texas, which is far bigger than Japan and yet was not enough to force extinction on all life on earth, only about 28% of all life perished.
To destroy earth as in to chatter it you literally need something the size of earth it self so that the gravitational field and center of gravity is significantly shifted to allow planet breaking. Otherwise you planet destruction is not possible.
For example 100 asteroids the size of texas would not be enough to even change earths orbit around the Sun. Simply because they do not have enough mass. (I'm assuming that these would be normal asteroids made of silica and maybe iron. Heavy elements are really rare on asteroids and the bigger the asteroid the rarer these elements become.)