Could part of Valyria’s drive to enslave more and more people have had something to do with feeding an enormous population of dragons? I know it’s never mentioned as such, but I always thought that feeding – for example – the 300 dragons during Garin’s last stand must have been a hell of a logistical problem.
While I’m sure slaves were sometimes fed to dragons, I’m pretty sure that would have been to terrify and intimidate the slaves into compliance, rather than as a primary foodsource.