With the screwing of the investments of players by nerfing whatever you want in this format you created the divide you are seeing however there are solutions.
1- issue WCs for nerfing. Easiest solution for retention. Players can choose to make other options and not play alchemy and not feel ripped off by prior investments in a format that is strictly pay to win.
2-start bringing pioneer(unnerfed) onto Arena. All cards are as is. You can still ban cards but other then that they are "eternal". You can also plan future anthologies or sets like "ravnica returns remastered", "Tarkir remastered ", "theros remastered" and it allows you to utilize the recent pioneer challenge decks you just printed for paper. Historic becomes pioneer in 2 years and players have their cards unnerved, and you make money on drafts . Win/win.