Our ancestors did produce their own vitamin C. But there are a bunch of deletions in our copy of the gene so it doesn't work anymore. Fixing it up wouldn't mean to move that far away from what it means to be humans.

Also if we decide that we don't need the gene, why not get rid of it completely? Why carry around a broken vitamin C gene? It seems stupid from a design perspective.

Vitamin C is just one example that's nice, because it's an ability that we lost in evolution but there are many small issues. For a lot of enzymes different species have an enzyme that serves the same function. Some of them however have better enzymes that work more efficiently. Yeast has had a lot more evolutionary cycles then us and might be simply better at a lot of housekeeping genes. For every gene we could search for the best one that's out there and exchange the human version with it.

Of course we will first do it on pigs that we want to eat. But if the pigs are much better when you gave them the best version of every housekeeping gene that's out there, some humans will also want to have the best ones.

I don't have very rational arguments for it, just the basic instict to conserve as much about our humanity as we know it as we can unless the cost is too high.

Then it sounds like you don't really want designer babies.

There's no 30 km/h running gene that you could possibly put somewhere. To get that outcome you would need to modify a bunch of genes and as a result have more fit people.