Assuming, of course, that the enemy doesn't have
A) the common sense
B) the ability
to shoot it and possibly blow it up. Though I suspect that if your enemy has B but not A, then the Colossus is the least of his worries.
B1) Not only is every army constructed differently, but so is every faction. Not everyone has the ability.
B2) By your argument one wouldn't be able to take artillery at all.
B3) To take it to it's extreme, you couldn't take anything because someone would have a counter since your argument exists in a vacuum. I specificly said "This works very well for static infantry heavy lists."
Sufficient infantry can ward off most attacks on artillery, especially if supported by Terrain or Vehicles to block Line of Sight. Setting up smartly is part of good generalship.
A1) Common Sense doesn't enter into it. We're talking about increasingly narrowing choices.
A2) The Colossus ignores most armor and all terrain. This removes the viability of simply hiding from it.
A3) This eliminates many options and forces the opponent to take action that he otherwise may have avoided.
A4) Supported by a sufficiently well built list, forcing the enemy to march into the jaws of your army can be a viable tactic. You need to be built around it though.
From a psychological point of view, eliminating an entire branch of options can cause an opponent to focus on removing it to the exclusion of other better choices, which is exactly what you want.