> Essentially, most of open source is slave labor
"slave labor" would imply a lack of freedom on behalf of the slave laborer.
I think there's something quite remarkable going on that is quite the contrary of what you say: People who work as software engineers in their day jobs are so starved of freedom, that they spend evenings and weekends doing more software engineering, i.e. more of what they're doing in their day jobs. Only now, they are actually happy doing it, because they're free. This freedom comes at a price, and this price is that they have to give away their work for free.
If paid software engineering jobs did a better job at fulfilling people's psychological needs on creative freedom, the drive for mastery, the desire to make a name for oneself through one's work, etc. there probably wouldn't be any open source, because the psychological needs that motivate it would be otherwise met.
If software engineering was better organized as a profession (like the way "labour" is organized when doing collective bargaining, or the way certain professions are organized like the legal profession) then maybe paid software engineering jobs would be better.
But we are totally disorganized as a profession and each individual seems to be perfectly ready to undercut others at every turn, e.g. being more accepting of bad working conditions, like lack of creative freedom, than the next guy. Studying harder for ridiculous leetcode interviews than the next guy. Putting in more hours than the next guy on hiring projects. Being more accepting than the next guy when employers refuse to give employees time for training on new technologies and instead doing it on weekends. When the work is actually enjoyable, being willing to do it for less money than the next guy. The equilibrium of that last dynamic is where the price is zero, and that's what open source is. A corollary of that is: Since all the enjoyable work gets done for free, there is no enjoyment left in any of the work that one can hope to get paid for. -- We have to stop doing this to each other. We have to stop the race to the bottom.
Open source, and not getting paid for it, is not the problem. It's the imperfect solution to a problem that, in a more perfect world, wouldn't exist in the first place.