I know it sounds odd, but when they swap an a/c like this (not in the schedule, but a "within two days on this flight only" kind of swap), it doesn't show up in the reservation where a res agent could tell there is a problem. It's only visible in ACS (airport check-in system). You're right, I'm sure 16C would have been better for you, but I'm guessing somebody was already seated in that aisle seat immediately behind the 2nd exit row. The flight left with every seat occupied, so I'm assuming there wasn't a lot of moving around room when they did the swap.

When swaps happen like this, it's within two days of departure and it's largely automated using one of Sabre's mysterious algorithms. Sabre just re-seats people as best it can (trying to disturb as few people as possible - sometimes not doing a very good job of it) and folks like yourself end up in a seat they weren't planning on.

Sorry it worked out that way. To answer the other question, the airline has software to notify folks when their flight numbers or times change, but nothing is in place to notify customers of a seat swap. My guess is that it's just not high enough on the "Does it generate revenue or reduce costs" list to be a priority right now. I'm sure there are several folks on this board who would change flights based on advance warning of a seat assignment change, but it's probably fair to say that the vast majority of the customers Alaska flies wouldn't change their flight based on a lost seat assignment. They might not be happy about it, but they'd still get on the airplane and take the trip.

Again, sorry it worked out this way. I hope the explanation at least helps you understand what's going on behind the scenes.