That nerf. Lemme calculate.
Suiton and Raiton are only single target abilities that lose damage.
With Raiton, you'll lose anywhere from 250-350 damage per minute, as you use the ability once a minute and crits are a thing.
With Suiton, you'll lose anywhere from 125-175 damage per minute, as you also use the ability once a minute, and crits are a thing.
That's 375 - 525 damage per minute. That's 6 - 9 DPS total. Not a big deal, but then you have the TP costs, which make Ninja more reliant on Paeon. I'll definitely be asking for it more.
Shadow Fang is 10 more TP every 20-ish seconds. Aeolian Edge is also 10 more TP every 20-ish seconds. That's 60 TP a minute I'm losing, 240 over 4 minutes. I'll feel just as starved as Monk will, using Invigorate as intelligently as possible. I'm usually around 200 TP when I use my 2nd Invigorate. I'll be fighting my TP to get to my 2nd Invigorate now, and I definitely won't be getting anywhere close to my 3rd.
This will prevent the Ninja from ever giving the Bard his Goad, because Ninja will be relying on the Bard to sing for him. In a MNK-NIN melee config, this will essentially change which melee is actually more reliant on Paeon.
If the Warrior volunteers to use Storm's Eye forever, then it costs the Ninja an extra 30 TP every 20 seconds instead of 20 TP. That's 360 more TP drained in a 4 minute encounter. It's essentially the same as letting a Monk use Fracture whenever it wants. The tradeoff isn't actually fair because abilities aren't the only source of melee damage.
Aeolian Edge is gonna be 60 TP for 320 potency. That's 5.333 potency per point TP.
Dancing Edge is 50 TP for 260 potency. That's 5.2 potency per point TP.
These numbers would look fair, like the ability to use Aeolian Edge is a fair trade of TP for damage, since it's more damage per point of TP. But if you consider autoattacks being 100 potency, and getting one every time you use an ability:
Damage during Aeolian Edge will be 420 potency.
Damage during Dancing Edge will be 360 potency.
420 damage over 60 TP is 7 potency per point of TP.
360 damage over 50 TP is 7.2 potency per point of TP.
Now it looks like using Dancing Edge is more efficient. It is. Don't forget the fact that Dancing Edge has a slashing resistance debuff attached to it. What this means is, that it's not efficient to run yourself out of TP using Aeolian Edge. Of course if you have an excess of TP, go ahead. What it means to have an excess of TP is that you're not gonna run out, like in a 3 minute fight, not that you have a large amount. It's not worth the Bard's damage to refresh TP drained by using excess Aeolian Edge. You may as well always use Dancing Edge because the amount of extra Paeon your Bard will have to sing isn't worth the extra damage you're doing.
This is in a 4 man scenario only. If the Bard is refreshing the TP of another melee, that's a whole bunch of extra usefulness that his Paeon gives, so he should obviously use it. Bard, however, isn't going to be using as much TP as the melee, so there will be "wasted" TP in the form of the Bard singing while at max TP if the Ninja asks for Paeon just for him.
tl;dr - They should have given Shadow Fang an 20 extra TP cost, and not given Aeolian Edge an extra cost, because math. Also, Monk should incorporate Fracture into his rotation, because Ninja is gonna be demanding that kinda TP anyway. RIP Bard damage.
Proposed Change: Do not increase the cost of Aeolian Edge by 10. If a TP cost must be imposed, increase it by 5. The extra TP saved can be applied to Shadow Fang. Make it cost 75 if a 5 TP cost is applied to Aeolian Edge, or make it cost 80 if Aeolian Edge is spared a TP cost increase.