I am a person that will play Frost Mage, no matter how viable it is always simply because I love the playstyle and role I feel I can play in a bigger online world as that spec. I always manage to clear mythic content, always get AOTC and clear fairly high keys, intend to keep pushing more.
Frost has been considered, for a while, to be less than viable in comparison to Fire to the point where people will not consider you for groups if you’re not Fire.
My advice, play what you love to play. If you love to play your warlock, you’ll make it work and do the content that you want to do. You’re better off playing the game as something you enjoy instead of something that is “viable” for a time. You will enjoy your time in game so much more.