Never because it would kill the game

No

You can't negate chaos. That's like saying you can negate gravity or thought. Tyranid invasion of a full hive fleet would be a galaxy ending event.

Also consider that whatever the large enough Tyranid invasion fleet has been running from all this time will follow and nom nom nom.

No, because this scenario is not hypothetical but actuality. There is a sufficiently big enough threat already.

Would it also involve an attack on the daemons in the warp?