I am strongly in favor of sub-classes in CRPGs.
However, they seem unneeded in A-RPGs.
Your own build effectively is your sub-class.
Blizzard just need to make sure enough skills etc. exist to allow players to make those sub-classes.
If the sub-classes are designed as detailed choices we make, they become like set items, reducing build diversity, rather than increasing them.
That said… it really depends on what is meant by sub-classes.
In many CPRGs sub-classes are just small rule changes, that does not limit your builds particularly. That could work okay. Although again, the setup of A-RPG builds kinda allows this to happen without sub-classes, so there is no benefit to it.
Also, it doesn’t have to be either/or.
Diablo 4 already has sub-classes.
That is for all intents and purposes what the Rogue class mechanism is.
Doesn’t mean Sorc needs the same.
I think sub-classes would fit the Druid really well though. Picking a “clan” or a “totem animal” that defines some aspects of your character (“Of the bear”, “of the Wolf”, “of the Raven” etc.)
Yep.
The thing sub-classes do is adding them as a hard choice. It can make sense for balancing, asking you to choose either Berserker or Champion. Instead of allowing you to go 50% of each.
Doesn’t seem relevant here though, if someone want to do the latter, let them.
Yeah, and that alone is enough reason Blizzard should not do Sub-classes 
Sub-classes in Pillars of Eternity 2 style can be fine, but also already supported in Diablo. Basically just “minor” changes to the class, like having to pick one of 4 passives for example. D4 could easily have that already, where you can only pick 1 or 2 of the main 4-8 passives at the end of the Passive Root system. Serving kinda the same purpose.
Also, I fear Ultimates will have this effect. And one of the reasons I am so much against Ultimate skills as a concept.
Big risk that if Sorcs have Ultimate skill A, B and C, then you effectively will have 3 sub-classes of sorcs, one for each Ultimate. Drastically reducing build diversity.
Just get rid of Ultimate skills as a design please.