Starting wis of 18 is not impossible. Seems that for a wizard, wis is arguably worth more than int (seeing how it also powers thunderwave), so as a human, elf or dwarf, you can probably afford a wis of 18 (which is really just 16 before the stat boost). So you begin with int16/wis18.
No, it's not impossible. It's just suboptimal except for the Orb trick, Thunderwave, and Cloud of Daggers.
Let's test your "wis is arguably worth more than int" theory.
First level Int 20 Wis 12 vs. Int 18 Wis 16 vs. Int 16 Wis 18 (same # of points).
Yes, Thunderwave can send a foe back 4 squares instead of 1 or 3, but that rarely is make or break (and of course, Thunderwave will not hit on as many foes with high Wis, so sending one foe back 0 or 4 and the other back 0 is typically not as good as sending two foes back 1 each, 9% of the time, this will happen, more with more foes). Yes, it could result in the foe being flanked or put into a wall or whatever, but usually not.
But the 5% or 10% difference on the attack hitting in the first place is pretty big:
As an example, 55% chance to hit with Int 20 Wis 12, 50% chance to hit with Int 18 Wis 16, 45% chance to hit with Int 16 Wis 18 using Cloud of Daggers (which is way Wisdom friendly).
.5 * 8.5 + .05 * 11 + 1 vs.
.45 * 7.5 + .05 * 10 + 3 vs.
.4 * 6.5 + .05 * 9 + 4
5.8 average damage vs. 6.875 vs. 7.05
Even though CoD is a huge Wisdom power, it does not average that much more damage. 1.25 points more per round vs. high Int. And at higher levels, it just gets closer in average damage. And in the case of minions, it matters not if the Wisdom is 12 or or 16 18.
On the other hand, Scorching Burst yields (same chances to hit as above):
1 foe: 4.8 vs. 3.875 vs. 3.05 (here, it's 1.75 more per round)
2 foes: 9.6 vs. 7.75 vs. 6.1
3 foes: 14.4 vs. 11.625 vs. 9.15
That's 24% more damage than option #2 and 57% more damage than option #3. When one considers Resist Fire 5, the more damage per shot, the better.
In fact, it's better for the 18 Wisdom Wizard to use Cloud of Dagger on one foe than it is to use Scorching Burst on two (except if they are minions).
However, that's just the At-Will powers. There are virtually no Wisdom Encounter and Daily powers. There, the high Int Wizard rules. So sure, the high Wis Wizard can lock down a foe (at real high levels) if he hits with an attack. But, he has to (generally) survive the low levels to get to that level of uberness.
Yup. One can have the one trick pony Wizard. I prefer a Wizard where most of his spells are real nice, not just one per encounter. Remember, an encounter might be with more than one group of foes too.
Too bad, the Wis 18 Wizard used his Orb on the Leader in the first group. What does he do on the Elite Solo in the second group? Answer: less damage, less times he hits, less control.
Note: Granted, a high Wisdom gives the Wizard a good Will save.