It depends on the numbers. If the invaders/conquerors are small in number, genocide isn't practical because there aren't enough hands to go around to run the farms. If invaders have the numerical advantage, either genocide or whole-scale displacement is the norm. Saxons were a minority with respect to Celts, just as Normans were a minority with respect to both, so subjugation was the strategy that made sense.
But there is also the cultural distinction as well: the invaders were generally herdsmen, not farmers.
They would want to keep the farmers right where they were, producing - and just make sure that the rent and obeisance went to them instead of the rulers they displaced.
The notion of "total war" is about as new as the notion of "equality."
Wars were fought, for the most part, while the common peasants sat and watched.
We must remember that, back then, "genocide" did not mean the wholesale slaughter of whole ethnic groups (as practiced by, e.g., Stalin or Pol Pot).
The captive men were perhaps emasculated, enslaved, or otherwise kept from reproducing through drudgery; their womenfolk were made into (perhaps willing) concubines. I would expect that, after three or four generations, there was consequently no longer any substantial genetic difference between the conquerors and the conquered.
Regards,