You add on a conditional to your flawed argument that makes no sense -- can you forsee the future and your crystal ball tells you that FFXIV will take hours to find/form a party? The armoury system is geared towards making it easier to fill the key roles... for example by combining Black Mage and White Mage into one class increases the number of healers dramatically, heck even THM can heal. MRD/PGL/GLA can all fill the role of tank and every class in the game can be a DPS even GLA could be spec'd to DPS.
You're ignoring one very important point:
The problem in grouping in any game (be it FFXI or FFXIV or WoW or anything) is not an issue of a lack of classes capable of performing a role, but a lack of PLAYERS who WANT to perform that role. Just because someone CAN heal doesn't mean the WANT to. Just because someone CAN tank doesn't mean they WANT to.
WoW is actually a great example of this:
Classes that can tank: Paladin, Warrior, Death Knight, Druid (Excluding situational tanks or weird tanking situations)
Classes that can heal: Priest, Druid, Shaman, Paladin
Classes that can do neither: Hunter, Rogue, Mage, Warlock
So by this logic, four of ten classes can heal and four of ten can tank. Two of them (Druid and Paladin) can heal -or- tank.
And yet how any raid groups do you see with 8/10 or 21/25 who have a handfull of these players but they're still looking for healers or tanks? Even if their gear could serve the purpose desired and even if respeccing was an option, most players would rather go with damage dealing classes. Sometimes you'll get a player who doesn't mind tanking or healing even though it's not what they'd -prefer- to do, but more often than not, asking someone who preferred a DD role to tank/heal instead is not typically met with agreement. Most players would rather disband the group and reflag as a damage dealer than agree to perform a support role and invite another DD to take their place.
In terms of FFXI, for the sake of argument, WAR and SAM could tank and DRG/mage and BLU could heal. But try convincing one to.
So while it may or may not take "hours" to build a party, the fact remains that just because several classes CAN heal or tank does not mean they WANT to. And unless you have a player who WANTS to heal and a player who WANTS to tank, you don't have a party.
The issue in FFXI was not a lack of PLD/NIN or WHM/RDM/SCH/SMN but a lack of players who wanted to perform the tank/healer role.