You are right that slavery is economically inefficient and that economists were one of the fiercest crusaders against slavery etc.

However: lack of demand is not a problem. People can create any amount of total nominal demand for basically free, as long as you have access to a printing press. (And with some minor caveats that's generally true in a gold standard setting, too.)

And even without that: your argumentation would suggest that as long as the slave-owners lavishly spend the money they save on wages, the economy would do just as well as without slavery. That's not the case; have a look at the arguments of the very economists you mention.