Wow, dejavu... I think I had this same argument on the ork board.
Indeed. And you failed to convince there either.
One thing you are forgetting, constantly, is spread. Fantasy is the ultimate example of spread. You cannot do the math for skavenslaves figthing swordmasters of Hoeth (the lowest of the low and one of the most powerfull units in the game in terms of close combat prowess for non-fantasy players) by calculation that 12 swordmasters would earn you 90 slaves and that after the odd 15 casualties (30 points), you'd kill about 8 and earn yourself 120 points. Why? Because, obviously, they RANK UP.
40K is less true in that regard, however, spread of power is still there. Orks hordes have more power than anything else in the game, agreed. However, the spread that power, due to the fact that it is contained in 120 odd ork boyz over a huge amount space, making the actual power per square inch, so to speak, not up to the total amount of power other armies/units can field. This power can be increased, of course, to a certain extent by bumping up your orks. However, any template weapons (which you'll need unless you're Necron/mech eldar, otherwise you have a though fight on your hands) will do massive amounts of damage. Combine that with the fact that orks aren't that fast, even with dual warpheads, and yes, orks are good, but no, they are not unstoppable. Good tyranid players should get the charge on them and can probably wipe out units by striking first with considerable force in one place.
Like others have said, if you clear your killzone or most of it of orks, and kill a nice number of orks on the next turn, close combat specialist can quite well take out ork units. Also, terain and all the other factors earlier mentioned work against the ork.
So basically, a fight may well turn out to be what you claim it to be, which means the ork player played his list up to strength, he had his wave and got to apply it. He will lose when the other player manages to not let him apply it in the way you just portrayed. How spectaculary fluffy.
You do not try to fight orks in a battle of attrition, you hit a few points with overwhelming force and round up from there.