First off, its not exactly clear how versatile Dolmen Gates are, but it seems reasonable to assume you only need a gate on one end (Otherwise their construction would not have been the crippling blow to the Old Ones it was). Second... Was it ever established how fast the intertialess drive is? I wouldn't be surpised if the Necrons still have the intertialess drive, but that even in their damaged state, the Dolmen Gates are still much faster, and the loss of them would probably put them in the same boat the Tau are in (Inability to deal with growth, as their ftl is so much slower than that of the imperiums)