I wouldn't feel secure letting my bag go on to SEA without me. Don't remember the layout of the SEA airport, but plenty of times I've seen a couple of lone bags just circling a long-abandoned conveyer. Easy enough for someone to say, "well, looks like no one claiming this one, guess I will."

At some point the airline is supposed to come by and pick up unclaimed bags off the belt, I guess, but I wouldn't put stock in them doing that and then just saying, "gee, guess he didn't make it, let's just hold his bag until he gets here." That's not really their business model.

Not to mention -- if you have an AA ticket but last leg is on Alaska, real possibility of "confusion" as to who has your bag (your confusion, or AA's, or Alaska's), and never finding it. Once I went to Europe on TWA and got re-routed for last leg on Swiss Air. Bag never came. I got a check but to this day I think the carrier switch was part of the reason it was lost. At that time (90s), even though I didn't ticket on Swiss Air, as the last carrier to my destination they were the ones I had to deal with on finding it (not) and reimbursement.