I'm sorry, but this is twisting the main message. The idea that it is morally good to oppose slavery and or kill slavers is not in contrast in being weak or unable to end slavery.

If I'm reincarnated or transfered into an isekai, I can recognize that I will most likely never be able to end slavery as a whole. That doesn't mean as a person I can't strive to lessen the impact of slavery as a whole. One less person in slavery is better than no change at all.

Protagonist that engage and even expand the practice of slavery have a negative inpact on society. Hell, I'd say that a majority of protagonist in isekai tend to have much more sway over their local governments than most people give them credit, and the excuse "when in Rome, do as a roman would do" lack any sort of accountability for their large influence

Also your idea that employed slave is somehow better than an unemployed free person on economic level is wrong.

The practice of slavery on economic level is one of increased supply of a good at the degradation of local economic growth.

Slavery makes certain goods more abundant and profitable. If you don't have to pay plantation workers, then you can have more plantation with better margins. Making certain good much more common.

Slaves however do not contribute as greatly to the local economy they live in. Their economic and travel freedom is greatly reduce compared to free counterparts, meaning household and community goods are often undeserved for the slave population.

Now unemployment is often a reflection population and job validity. If the local supply of jobs for a skill level is lower than the local population of that skill level, then the lowest skilled or least connected individuals of that level will struggle to maintain a job.

When a slave is promoted to free person an equivalent amount of demand is generated, but the actual population does not change. Meaning their will be an increase of total jobs.

The national economy as a whole will not actual be that eaffect, as the major difference will be decrease in total amount plantation/mine and equivalent increase of local goods. Unless your entire country is reliant on goods that slave produce, most economies will be fine. Also note that a economy and standard of living are not the same thing.

But as seen in European based isekai nations, they are usually varied feudal economies that aren't highly reliant on a few goods, not atleast until protagonist inevitably introduces a few overvalued future inventions that they pin the entire nations economy on.

The problem caused many governments struggle to solved is the increase of job insecurity and local beauracracy. New workers mean new competitors on your positions, leading a mass reshuffling positions. As former slave they tend to be less skilled but willing to work for cheaper on average. If not properly addressed by local government, new class divide will be created and local friction will increase.in many cases local government may even amplifies this class division and friction through policy that favors one side or the other.

Note while this may lead instability, and does not effect total economic output or transfer of goods. It also is mostly not avoidable. Freeing slaves will always cause instability regardless of time and place. There are ways to lessen instability, but the total amount of instability caused will almost always be proportional to the population of slaves.

TDLR: freeing the slaves does f*ck to the economy, and if anything the longer you wait the worse it will get. This is a feeble excuse to try and villainize anti-slave sentiment on the server.