Then the power shouldn't have such a high cast time.
It's the basic attack for a class designed around old school turret mage classes. The 1.5 sec cast time (which should never be described as a "high cast time"; your problem is that, as a basic attack, it *has* a cast time, not the length of it, which is the same length as a standard GCD) simply emphasizes the whole "turret" aspect that Sages have to operate around (and, yes, they do have to operate around a turret design because they're cast focused). Giving it a lower cast time would improve its outright DPS capability, while making it instant-cast would remove the turret nature. Keep in mind, if you spam Disturbance, you're going to deal more damage than someone else just spamming their basic attack (assuming similar gear, etc) so they're not intended to be directly equal. As I said before, Sages, thanks to having a drastically different resource paradigm than all of the other classes in game, have abilities that adhere to a different cost:effect paradigm.
And as a 'basic attack' it also fails because Seers and Balance specs really don't get anything from using it at all. Balance's spamming attack is TK throw.
First off, pretty much everyone has almost nothing that augments their basic attack so the fact that Seers don't get anything from it really doesn't mean much: they're healers, not DPS so it wouldn't make much sense for them to arbitrarily get something to improve it (Commandos just get Hammer Shot turned into a heal because it was a kewl way for them to get a free heal rather than just giving them a separate ability that was a free heal). Balance Sages still feasibly use Disturbance as a basic attack except that they have the *better* option of their talented and boosted TkT so there isn't much point. Noting that Balance doesn't really use Disturbance *ever* doesn't really mean anything; as I said multiple times before, Sages are different and, as such, Balance Sages got talents to "trade up" their basic attack. It has no bearing on Disturbance still being the nominal basic attack.
You're also failing to understand why I put "basic attack" in quotes, rather than just saying it without. Disturbance is *not* supposed to be mechanically identical to a basic attack. It's not even remotely close to behaving like one on the surface: it costs resources (no true basic attack does), it requires you to stand still (no true basic attack does), is a Force attack (no true basic attack is), can be interrupted (no true basic attack can be), and deals more than basic bonus damage (all true basic attacks deal bonus damage + weapon damage and that's all). It's *not* an actual basic attack, but it serves a similar role in the Sage's massive-resource-pool-for-a-turret-class paradigm, so it's *like* a basic attack for them.