I was shocked when SYTTD moved to Saturdays but it stands to reason. They show the same freaking Pnina Tornai dresses over and over again. It is the same dress but different bride every damn time.
That being said don't they have a bunch of shows about families with small children right now? OutDaughtered, Sweet Home Sextuplets, The Little Couple, etc. I don't think that is the sole reason the Duggars keep going.
I think if more of the 90DF couples manage to stay together and get pregnant we will see a spin off about them. My bets are on Deavan and Jihoon since they already have an extremely cute baby and are fairly likeable.