With unlimited Leves like you're proposing then anyone can get to level 60 without crafting a single item, you could just buy it all of the market and trade it in. That leaves the whole system open to massive abuse via RMT or people using rich alts to take a big short cut to 8 specialisations. In other words money becomes king. You like talking about bad game design, well there you have terrible game design. How exactly under your proposal would you deal with this problem? You can't just leave the game open to RMT abuse, it has to be addressed. DoM/DoW dont have this problem because you can't buy dead mobs off the market and trade them in for exp, for crafting though it's a real concern.
The challenge in crafting isn't making items, it's making HQ items. While leveling this is difficult because you don't have a vast plethora of cross class skills to choose from, nor do you have the CP to use them all. What you're asking for is the ability to turn in poor quality items for massive exp rewards. Yet again you're asking for it to made easier because you're a special little snowflake.
At level 50+ the scrip system is actually a pretty big grind, the exp isn't great but people want the gear which takes time to get, blue gear is meh, red gear is what it's all about and not only is it heavily gated by a weekly lockout but it take ages to earn it. That's a completely different situation to what you're asking for. We can't earn tomes on our lower level jobs, just like we can't earn scrip items on out lower level crafting classes. Turning in scrip items just for the exp is far less benificial and more time consuming than just knocking out normal high level crafts (in HQ) and selling them in terms of both gil and exp. People do it for the gear and in that sense it's a massive grind, not a short cut like you're asking for.
Why are you even comparing an RPG to an MMO? They're completely different games with completely different priorities and completely different challenges for both players and developers.