Every other champion with minions also has skills that work fine WITHOUT those minions. Let me give you some examples
Zyra - Without W still has QER.
Heimerdinger - Without Q still has WE and RW or RE.
Yorick - Without Ghouls being summoned still is a broken, badly designed mess.
Now look at how many abilities Azir can use without W. I'll save you the trouble: it's 1, and it's R. He cannot use Q or E without a soldier. His entire kit and theme fails to function if he does not have a soldier. With that in mind his soldiers cannot be harmed, as either they have enough health to basically be invincible OR they die and Azir no longer has a kit.
So if you want Azir's soldiers to be targettable ... the entire rest of his kit needs heavy buffs to the point where he could have ONLY QER and no soldiers and still function fine. And at that point you made a sand themed Zyra.