That makes sense when you consider how the manufacturing process goes. To have multiple blade type production at the same time they would need multiple furnaces, freezers, sputtering setups etc.I remember reading a message from Paul @ Connaught Shaving that the PPI factory operates in specific runs. It will be tooled up to deliver just Astra SPs for a period, then Gillette Platinums for example. You can see this by the transition across all the variants from 4 glue dots and double wrapping to twin glue dots and laser etching. Whilst overall stock of a particular variant may begin to get low, it's probably only a matter of time before they're due to be manufactured again.
Note this was specific to the PPI range so I'm not sure if this applies to others.