It wouldn't be the first time they updated characters. Every epic, rare and common 5-star in the game was initially 3-stars. Perhaps you forgot.
Keeping in mind that they should retain their classification and not be officially bumped up to mythic, the previously mentioned 5 star epic/rare/commons were given away in the same manner. So they were collected just as cheaply or cheaper in the case of the starting girls.
The strategy coming into play would be requiring players to question whether another 5/6 star non-legendary/mythic is worth collecting or building? Obviously, for those that whale the question is irrelevant, but such is always the case in these games.
I feel a larger library of functionally playable epic characters would benefit the game. If others still disagree, so be it.