They should. This is something of
@Omicron's worldbuilding that I don't understand.
In canon, the Esapada are nominal equals and ranked solely by raw spiritual power. (So a lower number is not
necessarily a better fighter than a high number, they just have a higher spiritual pressure.) This is a smart thing Aizen did. Under this system, Coyote's number does not mean he has de jure authority or privileges over Barragan and neither does it necessarily mean he could beat him in a fight. If you change it so that it means either, you would see Barragan challenging Coyote, then Aizen either loses the God of Time or the guy with even more spiritual power than him.
If Cirucci were to climb the ranks, Cirucci would not gain any more de jure authority of the Espada, her de facto authority would remain the same (de facto authority stems from power and having the number would not increase her power), and she would get no additional privileges.
It makes sense for Numeros to be able to challenge Espadas but not for Espadas to challenge other Espadas. If a Numero beats an Espada, Aizen loses an important subordinate and gains an even more important subordinate. This is a good thing for Aizen. If an Espada beats another Espada, he loses an important subordinate and he gains a
less important subordinate in the form of a Numero filling in the spot. (If the Numero was stronger than the dead Espada, it would've just challenged that Espada.) This is a bad thing for Aizen.
When a Numero beats an Espada, the average combat power of the Esapadas grows. When an Espada beats an Espada, the average combat power of the Espadas weakens. It makes no sense as far as I can see for Aizen to not have banned Espadas from challenging other Espadas for a lower number, especially when there are no privileges or incentives for an Espada of a higher number to gain a lower number.
If there were still some arrancars who wanted a lower number for the sake of having a lower number, it should be that the only thing they have to do is train and get stronger. If they have to beat another Espada, they have to train and get stronger then kill an Espada.
An Espada getting stronger is good for Aizen. An Espada getting stronger coupled with an Espada dying is not as good for Aizen.
It makes no sense for Cirucci to want a lower number. There are no tangible benefits to doing so and the way to getting a lower number should just be to get stronger, not to pointlessly eliminate one of Aizen's prime servants and replace it with something weaker.