Did God appoint Seth to be a replacement for his older brother Abel?
It’s probably better to think of Seth not as a replacement of Abel, but as the continuation of the godly line in the beginning of Genesis. What I mean by this is that it’s not that Seth replaced his brother. To our knowledge, Abel didn’t have any children.
However, Cain clearly represented the ungodly line because of his actions and the actions of his descendents. It was clear after God banished him from the garden and from the land around the garden to wander with a mark on his forehead that God could not use his descendents in the same way he could use Seth’s descendents.
The descendents of Seth seemed to be more godly than the descendents of Cain. So it is most likely that the idea here isn’t replacement of Abel but more of a continuation of those who seek to please God not only in their sacrifices and offerings but also in the way that they live.
When you trace the genealogies throughout the early parts of the book of Genesis, you see that Cain’s line looks a lot different than Seth’s. And Seth’s line tends to lean more toward the godly individuals we look at in the genealogy.
At least one example of these is Enoch. He walks with God for 365 years and then God takes him from the earth because of his righteousness. You don’t see anything close to that in the line of Cain. So it is most likely that the Bible is tracking two different ways of living, two different lines, one more godly and the other one is godly.