Thing is, helots could become free through military service in OTL Sparta, but too many free helots undermines the basic concept. For people to specialize in a task, they have to have their basic needs met, or else they'll have to do that first. Since there was little pay to be had in soldiering, for a Spartan to spend their life training, they had to own enough land worked by other people that they could have all their needs met without taking out time of their own to work. Thus, your labor force needs to produce a hefty surplus to support your class of leisured gentlemen; best way to do this is through quasi slavery. If too many helots become free, producing the necessary surplus to support professional soldiers becomes more difficult.
Secondly, Spartan inheritance laws would be a monkey on your back; since estates were divided between the heirs, it was easy for a Spartiate family to fall below the threshold necessary to pay for the rations from the communal mess, and thus citizen status. Forever. Wealthier families could then get a steal on this fragmented property, thus accumulating vast landholdings in very few hands, eating away at the citizen soldier body.
Maybe if there was some kind of ceiling on how much land one family could own, or if Sparta directly annexed more city states to distribute land and abate property fragmentation, you might be able to sustain the hoplite body better. Not wanting to risk losing citizens, Sparta developed a system in which small staffs of Spartan citizens -about 30- would use their expertise in drill, organization, and tactics to command large forces made up primarily of allies, helots, and mercenaries, rather than their own men. In this sense, the Spartans may actually be more of the aristocratic officer corps of their empire than the citizen soldiers.