I imagine they could make a mythical a grand prize or something? eg a big contest where the winner gets 1 mythical of their choosing or a lucky winner gets to design a new gun and gets the mythical version of it.
Doubt it would be in game where they give away hundreds of them.
Also receiving a mythical is kind of useless if it doesn’t work with your build. It can only be used to boost another item, not used as a legendary to upgrade.