My point is that Roman slavery was a lot more brutal than what was required by the lack of technology.

I beeeive (I cannot find hard data, although things like the Doomsday book should have recorded some snapshots of it) while there was a large serf population, it was not the "vast majority" as there were also lots of free peasants.

Yes, it was a hard life, but far better than their equivalents would have suffered under Roman rule.