I'm going to use FFXI as an example, because I believe we'll see a similar pattern of content locks for XIV. As expansions came out for XI there were story lines with content/access to content near the end of them. Then there was the difficulty with being able to participate in content you often needed at least some amount of gear from previous things to be on-par.

Gaining access/drops for a character in all the content (Sky/Sea/Dynamis/Limbus/Assault/ZNM/HNM/Salvage/BCNM/Einherjar/Meeble etc.) is just a daunting task for your second character... Which depending on your time might be months of work. However XI and XIV allow 1 character to perform multiple roles by changing classes. So you can continue to participate in the content as one class, while potentially getting equipment you can use for another.

That being said with the lock-out feature (Coil) and Token caps (Mythology) getting to "BiS" ilvl90 on one job, let alone on multiple will take time, and isn't exactly friendly. Even using Philosophy tomes to fully equip yourself in ilvl70 requires quite a bit of grind. Leveling up a second or third (or more) class (unless you enjoy Fate Grinding) takes significantly longer than the first, however it isn't without its benefits and you do not need full "BiS" equipment to participate without 'wasting everyone's time'.

tl;dr I don't think that Replay Value applies to MMOs, I think continuous play value is a better term.