Finding errors in a text is a useful exercise, but clearly a huge step down in terms of cognitive challenge from producing a high quality text from scratch. This isn't so much an alternative as it is just giving up on giving students intellectually challenging work.
> That's going to be what they do in their careers
I think this objection is not relevant. Calculators made pen-and-paper arithmetic on large numbers obsolete, but it turns out that the skills you build as a child doing pen-and-paper arithmetic are useful once you move on to more complex mathematics (that is, you learn the skill of executing a procedure on abstract symbols). Pen-and-paper arithmetic may be obsolete as a tool, but learning it is still useful. It's not easy to identify which "useless" skills are still useful as to learn as cognitive training, but I feel pretty confident that writing is one of them.