Hussar
Legend
That sums it up well. It's like highjacking flavor to legitimize the mechanics.
But, this is done all the time. Iconic creatures like giants, demons, devils and dragons have gone through HUGE revisions between editions. About the only similarity between a 1e dragon and a 3e dragon is the breath weapon type. Additional attacks, massive magical additions, immunities, you name it, dragon's have been mechanically redone. Giants, maybe not so much, just made a heck of a lot bigger.
Then again, look at the piercer and it's 3e counterpart, the darkmantle. To me, this is exactly the same as what was done to the rust monster. The piercer was a living trap. You walked under it, it fell down, maybe did damage and that was it. It couldn't do anything after it dropped. The darkmantle still drops from the ceiling, but, now it can fly, cause darkness and choke you to death.
Granted, I suppose, the name was changed, but, the basic idea of the creature is still very much the same. What Mearls did with the O-M is effectively the same thing. He took a pretty much useless as written creature, the 3e O-M and changed it to something that works. Not the same as it was in earlier editions, but, something that works.
If that's hijacking flavour, well, more power to him.