And for young devs, I 100% agree that slope is more important: the 1-3 years they're with your company will be ~half their careers or more, and they will hopefully grow quite a bit. Whenever you hire someone green you're betting almost exclusively on growth as opposed to current skillset.
For seniors (say 10 years and beyond), they're not going to be with you long enough for progress to make a huge difference, especially if they're decent to start out with - maybe that's not true 100% of the time, some people will stay around longer than expected and some people really do keep growing rapidly even into late career, but it's a reasonable guess. You're paying them primarily for the growth they've already done.