This is a betrayal of trust. It sounds dramatic, but its not. We are players of a collecting game and get cards by varied mixtures of time and money to obtain the parts to play a game. If a player is spending time and money on your game they should feel trust that the literal format will not be removed.
regardless of how you frame it, Alchemy Historic and Historic are not the same format and as more sets come out that divide between the two will grow ever wider.
You can talk of compensation for cards that are changed, but how will you compensate for killing a major format?
I do not want any part of Alchemy Historic if its replacing Historic and if my cards become mostly useless after rotation then I am not interested in getting more cards or skins for those cards. By extension I am not interested in daily grind to get these. And not a threat, just a fact, my motivation to keep playing.