Você está ignorando o fato de que cada mina contém uma pequena quantidade de handwavium . No universo Trek, aparentemente é possível extrair quantidades razoáveis de energia do próprio vácuo e a massa é fornecida à mina relevante por seus vizinhos (que então usam a energia que extraíram para criar mais massa):
Teoricamente, os cardassianos poderiam ter subjugado o sistema lançando coisas continuamente no campo minado, mas na prática, as minas podem operar indefinidamente, substituindo as minas quebradas e se recarregando.The replicator system was designed to accommodate a swarm detonation of up to twenty mines and still maintain the total shell. The replicator was a kludge of Cardassian and Starfleet types and included a raw-matter supply container able to contribute enough mass to build one-sixty-fifth of a complete mine. Mass for any one new mine was transported through the replicator's subspace emitters from as many mines away as was necessary. In a bucket-brigade system. As distributed over the entire shell, enough mass was stored to replace over 2,500 mines.
In the event the mass supply dwindled below 85 percent, the replicator sections were designed to extract particles from the zero-point vacuum domain to replenish the system. The threshold was set deliberately high because of the long lead time required to produce small numbers of particle pairs. - DS9: Technical Manual