Um, too bad that they can't spend a little time doing fresh content just so they can gain access to more fresh content? Their choice. If they would rather play a sandbox game, then this game is not designed with them in mind, nor will it EVER be.
Funny, the 'zergrush' method IS doing story in Heavensward. So the point is moot.
18 hours is based on several accounts in this thread, if you read back. and that is at patch 2.55.
Itemized, that comes down to 9 hours story, 9 hours grind.
Yoshida's statement was that MSQ will eliminate the need to grind for XP or gear, because MSQ will provide more than two times as much XP AND ilvl 90+ gear, allowing you to easily coast to Ishgard explicitly doing MSQ.
Grind eliminated, level 1 to 50 (with i90+ gear) would then be 9 hours flat.
Your special scenario ruled out needing to level from 1 to 30, so I did an estimate and said half as much time is needed to catch up for a level 30 player. So 4.5 hours.
Don't worry, we will have a large influx of data beginning the 23rd. And we are sure to get plenty of those newbies being carried by friends. Which is also a shame, as they won't learn jobs properly, but I digress.
And if they play 20 minutes a day it will take two months. But neither of these time=progress sums really mean anything. To bring it up is just a distraction. People decide on whatever time they can commit, saying someone who plays less is owed more advantages is just plain silly.
Fun fact: If you want DRK, you had better be leveling MRD and GLA or you will suck. I mean. suck. No provoke on a tank? really? No extra cooldowns for survival? Skipping to DRK would be suicidal and punishing other players trying to "deal with you".
You would WANT to level a tank if your goal is DRK. You would likewise want to level a healer if AST is your goal, or LNC/ARC if you want Machinist. These really are advanced jobs in the sense that they lean on what you learned in the other jobs of the same role.
If the player were smart and wanted fast leveling for MAC, they would level it to 50 and THEN begin the 3.0 MSQ. Yoshida already said the new MSQ will be more than sufficient to get a class from 50 to 60. So there's that.