If/when rotation happens, I don't think you will be able to liquefy old cards

I think if/when rotation happens, it doesn't make any economical sense for you to able to liquefy cards that are not in rotation.

Suppose that every expansion, you are able to receive X value in resources based on the events/income the game gives. And suppose that when a rotation happens, you are able to liquefy the set that had just rotated out. Assuming that rotation gives you 30% of the value of X in vials back, this means you will be able to get X+0.3X = 1.3X value in resources. This holds for all future rotations, but when the first set in which you were able to get 1.3X value rotates, then for all future expansions after that, you will be able to get 0.3*1.3 = 0.39X extra resources, or 1.39X value in future rotations. So as you can see, the amount of total expansion resources you get will continue to climb.

This means that long time players have increasingly less reason to spend if they allow this, which is the opposite of what Cygames wants you to do. Long time players should be the players most likely to spend, for they're the ones who are most invested in the game. Playtime should equate with increased spending as you become more attached to the game and the story, decks, leaders, etc.

The only way then to counteract this would be to reduce income through less events every expansion, which would also not be good. This would punish new players unfairly, who do not have cards to liquefy. Instead, if they just keep the resource value constant at X for every expansion without being allowed to liquefy, then there will be no resource inflation.

They can then adjust X through events, which they can make more or less generous to hit a sweet spot in which you are incentivized to spend some money to get what you want. If it's too generous, then you would not spend money, while if it's too stingy, you would just quit and also not spend money.