But many people request content from other books. Obviously yes, injecting every book into this game would be at best improbable in scope and more likely impossible. But people still desire that because they want a full dnd 5e game. But the PHB is very very possible, without any need for updates or DLC to inject missing content. Personally I feel like you are overestimating how hard it would be to add subclasses when the mechanics for most of them are done, in fact I'd look at games like Pathfinder that have included a LOT of their source material which I feel amounts to more than the PHB in content easily. In fact, if we don't get more than the PHB (not counting what we already got like Gith) I will be surprised. Larian is the type to add all they want and stop when they start to get too far, like implementing Dinosaurs.
