I kinda have to agree with Warren on this. There are definitely uses for gated content and I'm a believer that you should have to play the game to, y'know, play the game. (Even though I myself really really hate trying to level my BAL alt up to 60 on multiple IC classes just for glamour usage and flying in heavensward zones.)
That said, just because future expansions may provide gated content, does not necessarily mean that they have to be gated behind previous expansions. 4.0 could provide a horizontal storyline to 3.0 stemming from 2.55 or another patch. FFXI is a great example: XI's first expansion Rise of the Zilart expanded upon the MSQ and extended the game past the Shadow Lord final boss and added Sky content that was accessible parallel to that storyline. Then, XI's second expansion went back and actually started at Lv25 with the Promyvions, creating an intricate different side story that spun off from the MSQ's and exponentially rose in level requirements the further into the questchain you delved. Something similar could just as easily be done in XIV.
That said, just because future expansions may provide gated content, does not necessarily mean that they have to be gated behind previous expansions. 4.0 could provide a horizontal storyline to 3.0 stemming from 2.55 or another patch. FFXI is a great example: XI's first expansion Rise of the Zilart expanded upon the MSQ and extended the game past the Shadow Lord final boss and added Sky content that was accessible parallel to that storyline. Then, XI's second expansion went back and actually started at Lv25 with the Promyvions, creating an intricate different side story that spun off from the MSQ's and exponentially rose in level requirements the further into the questchain you delved. Something similar could just as easily be done in XIV.