I think we only need one faceless horde of communication devoid xenos, really.
Two counterpoints:
- The Necrons hardly qualify as "horde"; quite the opposite, actually.
- The Tyranids are basically just animals.
If anyone felt an overwhelming urge to ruin a specific faction, out of the two, I would've preferred the Tyranids to go the way of the dodo. They contribute virtually nothing a random xenos species going hungry hungry hippos wouldn't be able to pull off with their hands tied behind their back.
The only thing remotely interesting about the Tyranids is genestealer cults.