This is a totally different hypothetical scenario than what is happening with GW2. You're describing the addition of a vendor that sells gear that is below the current best-in-slot in order to bridge a gap between best-in-slot gear and something lower witihn a game that's designed such that you don't need specfic gear to complete content. Ascended gear is better than the current best-in-slot, exclusively is able to hold upgrades that are necessary for completing certain content, and "bridges the gap" between the previous best-in-slot gear and a prestigous best-in-slot gear by itself being a new best-in-slot gear.
If you wanted to make a GW analogy, it would be like the developers adding a third god realm of several levels with deeper levels containing enemies that applied a new type of Spectral Agony which could only be protected against by upgrading a newly introduced set of gear that universally has better stats than what was previously the best. This analogy doesn't have the new gear acting as a bridge to anything, but nothing in GW was designed like the legendaries of GW2.
As of the moment that Agony is introduced to the game, yes. The new dungeons will eventually put players against stacks of Agony that can't be overcome without the use of ascended gear, and thus you need ascended gear in order to play that content, which means you need ascended gear in order to play all of the content in the game. If instead they don't throw sufficient Agony into the new dungeon to require players to move up to ascended armor, then what was the point with adding it whatsoever? It doesn't serve as a bridge and you don't need it to play any of the content, so why add such a pointless tier at this stage in the game?