No they aren't. The gameplay in Dungeons are vastly different. More of an emphasis on exploration and puzzles and more time commitment. Strikes are for casual farming. Both are needed.

Eh, I see Dungeons as a step up from Strikes. Strikes are good for killing time with randoms you don’t need to talk to. I liked in Taken King when Strikes started having some simple but fun mechanics instead of just being a shooting gallery, but Dungeons are a bit too complex to just replace Strikes. Strikes are necessary to give PvE players something simple to grind. But don’t get me wrong, Dungeons are also great for those that want a Raid-like experience with less time constraints and less people. I think both is good