My favorite part about this is the portion of the people who do this and haven't even finished leveling a class/job yet. There is ALWAYS something to do, even if that something doesn't really provide much of a benefit to you to do. For example, let's say that, hypothetically, you have completed all of the content in the game, farmed everything worth farming, collected everything worth collecting, leveled every class/job to max level, the hypothetical "everything" if you will to the point where you can no longer make any progress. As far as making progress goes, there's nothing left to do. However, there's still a LOT you can do even once your progress can no longer move forward. You can still gather materials, you can still run dungeons/trials/raids/guildhests/etc, you can still craft, you can still stroll the streets of Ul'dah selling Chocobo Feathers, you can still sit there and stare at people as they go about their own business, you can still visit the newbies and watch them begin their adventures in Eorzea (and preferably offer them a helping hand every now and then), you can explore and try to find secrets and details you might have missed, you can even.... dare I say it! Dare I do! You can even make a new character and start from scratch! After all, we have what? 40 character slots overall if I remember correctly? Plenty to do. You just have to be willing to do the do!
As for the OP and his suggestion, I will have to agree with some of the others here in that I don't think this should be a thing. Crafting and Gathering can produce some of the strongest gear to use while leveling and gearing. This would throw the entire market of the game out of balance to just be handed "free" levels and gear to be used for Crafters/Gatherers. Even ignoring the market aspect of why this is a bad idea, there's a significant amount to learn and master about the DoH/L classes/jobs (Crafting, at least, I haven't glimpsed Gathering in this game yet) that doesn't exist within other MMOs, such as having to have appropriate gear on and ACTUALLY making the item manually at least once before you can take the easier route of Quick Synthesis. There's even the chance that the craft could fail, and thus you lose the items, meaning you have to take care in how you handle the materials as you craft (much like you would in real life).
Is it a grind to level Crafters and Gatherers? Yes, it certainly can be, but you're playing an MMO for Hydaelyn's sake. MMOs are all about the grind. If you remove all of the grind by offering, albeit optional, paid conveniences to skip everything, then it's not really a game anymore. Rather, it becomes a Pay-to-Win pile of BS.