There is an unresolvable tension between the quantitative and the qualitative, and it's hard to imagine a more extreme example than erasing ineffable human dignity in order to turn a human into a commodity.
Let's flip this around to today: imagine a hypothetical future 100 years from now, that honors the qualitative dignity of all animal life, and views factory farming with the same horror that we view slavery now. From this moral vantage point (even as a meat eater, I concede this position the high ground), a phased approach seems appalling.
We currently slaughter roughly one Holocaust's worth of animals every hour; how about reducing it to one Holocaust per two hours? Maybe a new law to give cows an hour in the sunshine before they're herded back into pens? What about voluntary buyback programs? Taxes on meat to subsidize cheaper vegetables? These are all quantitative solutions to a qualitative problem: either cows have inalienable value as conscious beings, or they don't; and if they do, shouldn't we stop at nothing to defend that sacred value and eliminate all human-caused animal suffering, rather than wishy-washy incrementalism?
The unfortunate reality then and now is that we're stuck in quantitative games, directly or indirectly. There are no shortage of highly motivated consumers and producers who benefit from factory farming, just as as there was for slavery. There was clear justification to go to war to end slavery, to forcibly grind the game entirely to a halt; but it was not without cost, not only in lives lost, but in a consequential cultural rift that persists to this day. I'm both ignorant and curious how culture played out in those societies that use phased approaches; do they have equivalents to the "Southern Strategy" and Nixon's racially-motivated War on Drugs (which is rightly, I think, compared to modern-day slavery)?
I don't claim to know the answers to any of these things; but until a given moral phase-shift takes place, it's not always obvious how to get there, and a hardline stance can run the risk that the system kicks back, opponents are strengthened and galvanized, and nothing is accomplished.
(Yes, I'm aware of the irony and offense of invoking a comparison to farm animals in a discussion of slavery, when slaves were once viewed as livestock. I'm attempting to draw the opposite point: of humanizing animals, in the same way that we now humanize all homo sapiens regardless of ethnicity.)