They should just delete AST

[Mortex]

Here the math. Expanded balance is straight up 5% extra dps.
Expanded arrow is 5% reduced GCDs and auto-attacks, which translates to 1/0.95 = 5.3% extra dps in regards to GCDs and auto-attacks only.
Expanded spear is 5% extra crit rate. Assuming the worst case scenario at 364 base crit, we're comparing (1*0.95+1.4*0.05) to (1*0.9 + 1.4*0.1), which is 1.96% increased damage on average. However, spear only lasts 20 seconds. So to compare them, spear (1.96%*20 = 39.2) is worth less than a third of a balance (5%*30 = 150). Again, this is only if everyone in your party has no crit bonuses. With higher crit, the better spear becomes.

[Seraphor]

Expanded arrow is 5% reduced GCDs and auto-attacks, which translates to 1/0.95 = 5.3% extra dps in regards to GCDs and auto-attacks only.

Entirely theoretical, and only applies when protracted over a long period of time.
In a 30s window you’ll have about 14 GCD’s max, or 12 if you haven’t stacked any Speed materia.
That’s not enough time to reap the benefits of a 5% increased speed buff. Your 13th/15th GCD at the end of that 30s window will come a second or two quicker, and not even a whole GCD quicker. That’s it. You’d need to be able to fit 20 GCD’s into that window to gain one whole extra GCD, and only Ninja could do that, but that would cause clipping and therefore cancel out all benefit of it.

If it was a permanent buff throughout a fight it would make a difference.