I think the reason why you don't meet Delwyn is very easy.
With cost of 5 Mana he is a too expensive card for low Mana-Matches, when Water is available it is better to use one of the Water Summoner who add also Magic Buff but cost 2 Mana less or add +1 Speed and + 1 Life to the same Mana cost, this is IMO more important than the possibility to be able to use dragons (in low Mana-Battles).

In high Mana-Battles you will just use Byzantine Kitty who is IMO (in high Mana-Battles) nearly always better than Delwyn, even in ruleset without healing what take a lot away from Byzantine Kitty I think that true Strike and +2 Speed is much more worth than + 1 Magic.

So yes Delwyn is the card for the player who are looking for a cheap dragon summoner, f.e. for the price of one Byzantine kitty max-level in the market you can buy arround 5 Delwyn or in other words you pay for a Delwyn max-level only 20% of the price from a Byzantine Kitty. Yes Archmage is much more expansive than Byzantine Kitty but this is IMO a question of rarity as it exist 1255 Byzantine Kitty but only 87 Archmage Arius (regular foil)