Diablo is getting a board game and tabletop RPG that will “continue the lore and story” of the video games
(www.rockpapershotgun.com)
"Beneath the Monolith" was also put out by the same company that produced the original setting with its bespoke system (Numenera/Cypher System). They just know which way the wind blows and strive to maximize their profits.
Multiclassing is an optional rule in D&D5e, not allowing it should not be controversial.