Multiclassing has been part of D&D since at the very least 2e. There's no reason to remove it if you're staying true to the idea of the tabletop ruleset. You're not forced to do anything.
This is coming from someone that does not multiclass 95% of the time. In my party, my main party right now, the only one that's multiclassed is Astarion. My sorcerer is not, and none of my other saves' main characters are either.
This is a PC RPG game, not a tabletop PC port. The TT game has an entirely different dynamic, including the obligatory multiplayer campaigns, the DM's, and the fact it has no manifested gameplay in a real game environment like a videogame does. I don't care that it's had multiclassing in the rulebook, it can even stay in as an option for versatility, but it should never eclipse the pure classes, over which the entirety of this game's dialogue and reactivity is built.
You may be fine with some melee half paladin parading as warlock, but people who roll warlock for the actual class description in character creation expect a competitive full class, not some pieces meant to be cobbled together with another class.
A game with some IP inspiration is not a religion. It does not require strict scriptural adherence to some holy text. It is a PC singleplayer RPG game and it is free to deviate in design as it best suits it for the balance and gameplay experience of the game.
