What will be the purpose of collecting juice in this post-stat-system-changed world? A lot of this game's identity comes from this mechanic and I think it would be waste to remove game mechanics which revolve around spending acquired juice so it follows that I think there should be some equally important game mechanic focusing on the exchange of juice. So perhaps you could keep the fountains in-game but heavily nerf them? They may provide additional stat boosts that the player gets to spend their extra juice on in order to further juice their character out. It seems like you could manage this alongside your new system by making fountain upgrades become much more expensive much faster so that the player will have less economy to work with (but still an economy nevertheless); making subsequent upgrades more expensive will also stretch fountain use out over the course of the entire game. You will also now have multiple ways for players to interact with the new stats as well. By adding your new stats to the old juice-fountain-stat-pool, players will be able to "demo" different builds by reallocating their juice economy into different stats.
My vision of the game assumes that the 'end goal' of the player is to become absolutely maxxed out. An end-game player will have experienced all content in the game and then some in order to max out their juice fountain and be able to play with any weapon class they choose as a consequence of solving the game with whatever strengths they initially built.