Single cards sell for higher BCX than combined cards. So if you are a seller of cards it makes sense to keep separate.
But low BCX cards are useless in battle so if you want to win and earn you need to combine.
Maybe some players already have a full maxed out set so then keep the singles to sell.
I hear that side and can see that's the strategy for many. I think I alluded to that, or meant to, but removed it.
I feel that there must be some sort of intersection where collectively combining cards raises their floor value as singles and combined copies. In this happy medium, it could theoretically be more profitable in the long term. With so many wanting to keep singles for the scenario you described, it feels like a race to zero with the over-saturation. There typically aren't enough players to soak them up at meaningful prices. It also feels like selling singles for more can sometimes not be how it pans out, especially during bearish conditions. I think it's more in our control than we give ourselves credit for.