That analogy doesn't work at all because the Sow produces a finite (and rather small at that) number of piglets over a given timespan.
It's more akin to you getting a piglet/sow elsewhere. Now your piglet/sow need is satisfied and you won't buy anything from this farmer.
(Edit: And even then you took that piglet/sow away somewhere else, reducing supply there, which will make it more likely for this farmer to get a sale in the future.)
What's also kinda wild is how those plans often have 0 interest rate as long as you're able to pay the installments on time. Which means in theory you MAKE money by using them because you can earn interest with that money in the meantime.
It ALSO means they know the people using those services are so bad with money that they can sustain themselves (and make a nice profit) purely by their clients failing to pay on time and then selling the debt to debt collectors. It's absolutely disgusting how predatory this is, making their money mostly on the people who'd need such a system the most (and to a smaller amount, on people who don't care).