Alts are only an option if you're a no lifer really. For the vast majority there's plenty to do every week between Coil, Capping Myth and leveling other classes.

Basically the idea of Myth cap per job is a terrible idea, it's counter intuitive to how the armory system works and will only benefit those who have much higher than average play times. I don't think it's understood that a lot of players simply don't want to have to cap any more than 300 tomes per week. Also, the HUGE problem is that you'd be restricted to farming for myth on the job that you went into the dungeon as. There's not really any way around that, it would certainly be locked to the job, if they could even implement the idea.

If you can level another character and have it ready for coil in 2 days then good for you, most people don't heave the bandwidth to do that. It's actually a perfectly fine solution if you find yourself running out of content every week. However, there's not much from stopping you doing this on your main too, at least to the point where the job is ready for T1-4. Beyond that, then yeah, it's pretty clear that we're not supposed to have multiple jobs decked out in iLvL90 gear. For the moment I'm perfectly fine with that notion.