Prismatic overshadows the other subclasses so hard. Transcendence is easily accessible, infinitely sustainable and can be chained into a super which you chain back into Transcendence which you chain into another super ad nauseam.
Even if you ignore Transcendence, Prismatic still makes other subclass gameplay loops completely obsolete (why would I bother using Combination Blow on Arc Hunter when the Prismatic version is better in basically every way?)
The only Light/Dark subclass builds that have survived are the very strongest, niche PvP builds, or the ones that give you access to movement tech like Well/Shatterskating or Grapple.
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I can personally still find plenty of reason to use the old subclasses. My whirlwind maelstrom build on hunter doesn't work without the good old strand subclass, for example. I have a build based around it for both pve and pvp. Prismatic is still really good, but before we account for transcendence I would say that the two are about even. I think it may just come down to a lack of imagination, or the destiny community's toxic desire to do whats "optimal," even if that optimal play isn't really all that optimized. Once you know how to play around with buildcrafting enough, I think most people are gonna find that the old subclasses haven't really been left that far behind in either mode.