The thing that made slavery less profitable on the eyes of the State in vicky2, IRL and i believe will be the cause for it here too (in part at least) is simple: slaves don't pay taxes.
Slavery is inherently a institution that strenghtens the nobility, as they will have more power on their hands (they literally have someone's life after all) and lowers the revenue of central authority (i never read about large-scale "slave tax" and most of the money that States always got was from the masses, the nobility, because of obvious reasons, rarely got the grunt of taxation)
Wich was the reason many raced to abolish it in vicky2, slaves don't get salaries in that game so they don't pay taxes so you are poorer while the aristocrats are richer since there is a portion of their production force that you simply cannot tax.
There is also the issue that slave production, such as that of the serfs, is inherently lower than that of free working farmers and labourers, because they are working to fill quotas and not their personal gains, and also all the socioeconomical problems that such a stratified economy bring. All and all, slavery trades final output for control over the producers, wich is something viable, just not the optimal strategy.
On the issue of industrial slavery, it depends on how you look at it. It certainly isn't something unviable as the nazis used it a lot to create the machinery for their war effort, it just works in a very different way than industrialization is accostumed to work because of the precise conditions in wich industrial economies where born, wich where in more and more free-economy societies like England, the U.S., Low Countries etc.
To make a slave industrial economy viable, you would need to breed a type of system probably mainly exportation foccused or in wich the industry is aimed at supplying the army or other base needs, since you won't need the workers of your industry being consumers (they aren't the target audience of production forces) and will still be able to turn a profit.
At the end, its just a question of what you want to do, the ends shape the means a lot, and since paradox is going with a sandbox style game as we can see by the trend of their announcements i believe industrial slavery might be a possibility.