As a proud Sunsinger main (ever since I unlocked the subclass, in fact), I'm both excited about the new subclasses (Dawnblade, Sentinel, Arcstrider), and concerned. We have existing material in the lore which references the Sunsinger, Defender, and Bladedancer subclasses, so I can't honestly find a valid reason why they'd be gone for good. I know some might say, "The Cabal cut us off from the Traveler's light, dummy! That's why we lose our powers! Haven't you been paying any attention?!". However, it's clear we regain them, as evidenced by the main story's plot. Plus, the Striker and Gunslinger subclasses are making a return, so what gives? Osiris is generally assumed to be a Sunsinger, but to be fair, that may never have been explicitly stated. What shall his DLC bring, I wonder? Even if the specific [i]title[/i] of a subclass may be just semantics, certain abilities have basis in-lore. For example, Radiance, and Fireborn in particular. The grimoire card for Radiance states the following: "A Warlock in a state of Radiance threatens to slip beyond the bonds of the material, shrugging off physical harm, channeling a torrent of abilities. Some may learn to elevate nearby Guardians, gifting them with power. Others, entranced by the Ghosts' power to reach beyond death, may learn to pluck themselves out of nothingness like the phoenix of ancient myth." If a Warlock can no longer self-resurrect in Destiny 2, how is that justified lore-wise? TL, DR: Some things about current subclasses are part of the lore (e.g. Fireborn Radiance), so it doesn't make sense to remove them in Destiny 2.
Simple, the new sub classes are actually new sub classes. Let me explain, we lost our powers so we have to learn new powers.