And these days, one can simply do better. If I play a vanilla 2/2 for 2, something went decidedly wrong or I am hyperaggressive.
What kind of card in past sets were 2/2 for 2 with : a. a combat ability (trample here) b. a pump/lib manipulation ability. The dual color for me is not a factor since there is good mana fixing in this set.
I'm not saying they never existed, but to me it's not as common as you say it is. Nor is that card as sucky as you say it is. Maybe it's not as strong as I say it is... which wouldn't surprise me since I'm just getting back into it and never been a pro anyway ! Maybe my problem is that I say it's stronger that it really is... but by reading you I'm under the impression that you say it's worst than what it is. Let's compare it to other cards with 2 CMC and trample (I didn't bother to check other combat ability such as Firststrike, but my point stands since most of the other are for other purpose than combat (like Thalia)) :
Avatar of the Resolute is also 3/2 for 2CMC with trample and can get better. Excellent indeed. Maybe better than Grim Flayer. But I'll take the Lib Manipulation instead personally, especially since it has delirium and add something to that mechanic.
Furyblade Vampire is 1/2 trample. Her ability is nice for a madness deck, but nothing shining.
Goblin Deathraider is good for aggro, 3/1.
Hellspark Elemental has Unearth and Haste, so it's kind of a different role.
Hunted Horror is cool, but I didn't play constructed at that time (Standard) so I have no idea if it was used.
Kargan Dragonlord is nice, but not the same kind, and not broken.
Kavu Predator can become very big indeed.
Lotleth Troll was used alot. Very good card indeed. Comparable as a beatstick since it can get even stronger in it's own block/standard.
Etc.
If you go in more older cards, like Pygmy Razorback or Satyr Rambler, you have 2/1 trample for 2CMC, with nothing else. I get it that Grim Flayer is Mythic, but in the end in constructed standard, it doesn't change squat because people will buy the card. The only thing is does is more cash-money for Wizard of the Coast and such.
Is that card better than Goyf ? Of course not... But I still think it does the same kind of job : building up throughout the game and serving as a beatstick. The fact it has lib manipulation and trample only makes it awesome (still not better than Goyf). Even if I agree with you that it's not that groundbreaking as I made it to be, I still think you are downplaying this. Price of the card doesn't mean squat. When Snapcaster came out it wasn't even near the price it is now and I loaded up on him. Then people understood back what Flashback meant and it went up. We'll have to wait and see.
And like I said I don't think I'm a decliner: I like the new set (even if colorless becoming a color is kind of lame).