Not counting the Bozja areas, most of the fights you listed, you can't even get to the enemies rear, so that have to be omni positional. As for the Bozja areas, the only reason they don't have positionals is in the case you have a melee tanking an enemy, don't want to lose that damage just because you were potentially forced into that role now (I've tanked DR as a Monk before with a random group, so it does happen). So really, it is more out of the content's design that the decision was made.

The only case is Ruby Weapon, where, in the second half, obviously no positionals, however I suspect that has to do with the fact the boss doesn't move has to do with the lack of positionals.

As for the arrows on the side of target circles, I am 99% sure the devs said it was so that poeple could more easily identify which way the boss was facing to aid in dodging cleaves etc. It also helps in avoiding mobs as you can now see which way that elemental is facing.

As for positionals in general, there is no reason they should go and the devs have shown no intention of doing so. With that said, there is no reason you couldn't have jobs that rely on positionals more than others. Monk is the high end, the rest are all lower and, depending on how he devs want to do it, with only Monk being highly positional dependant, the other 3 being less, I can see Reaper either being, no positionals to appease the poeple that hate them, or they could make it highly positional based as an alternative to Monk for people that like that playstyle. We should find out soon though.

However, there is still a balance that must be kept between being too reliant on positionals and them being meaningless. ARR Dragoon was too reliant on them, with Heavy Thrust not providing its buff if you miss the flank and Impulse Drive not comboiong into Disembowel if you miss the rear. Messing with your rotation in that regards is too far. However, something like Dragoon's Raiden Thrust is a good example of it done right (as someone mentioned earlier). As a random idea, they could bring back a gauge that fills as you hit positionals and above a certain threshold, they change your combo, similar to SB Machinist and heat. Have a dump skill to drain the gauge and have no penalty if you happen to max it. If you miss a positional, it could reduce the gauge to incentivice you to hit them and then Anatman could be a tool used in downtime to slowly fill the gauge, similar to Samurai's Meditate. That whole thing was pieced together as I was typing...really haven't though too much about it so criticise away.

With many many people that do enjoy positionals, it would be a shame to see them go, however, the job that relies on them should be the one that gets rewarded the most for it.