I appreciate the effort in trying to justify this, but this doesn't make any sense. If Plarium wanted to provide a legendary to help players get acquainted to core game mechanics then it could have been a single day like Scyl. Alternatively, remove the XP Brews and put the shards there.
No, it was a conscious decision to specifically target these 2 items. This means the motivation was not to provide a new legendary, but rather to remove these valuable rewards that must have been impacting sales.
The purpose of the shards is to drive players to participate in so much more, whether it is summon rush, champion chase, 2x summons, 10x summons, guaranteed champions, fusion/fragment events, etc. The shard in the daily login encourages players to participate in the game, which in turn adds value.
Having to wait 4 months for a legendary does not drive the player to participate in anything, other than to just log in each day, and then log out.