Argammon,
You are right about the specialist thing of eagles, but still:
- on a small map I would not build demons, nor would the opponent, so no need for Pegasi+Eagle to fight them. Havy Cavalry, sea serpents, ... play the game there. With new suggestions for demons, demons will not show up at all anymore as you simply won't have enough cities to both produce enough demons AND produce enough other one/two turn units to defend the cities and fill the demon's "waiting hours"...
- on large maps I haven't seen any player produce elves as a strategy yet to massively crush his opponent.
But you are right, eagles are weak against land units. Still it is more on these large maps that I was reflecting: flying units are mastering most games on large maps and even then it is still possible to diversify with land units as you will own many more cities than on a small map. With an opponent betting on large demon stacks I would definitely think of countering him with Pegasi+Eagle.
If the P+E still doesn't convince (every game is different), in many occasions Pegasi only or Pegasi combined with other would still do the (cheaper) job against demons.
Also curious to see if the 55 max is a max for swarming or really a cap incl. leadership/morale/blessing.